Manufacture of Arsenic Acid

Europaisches Patentamt European Patent Office © Publication number: 0 213 244 Office europeen des brevets A1

EUROPEAN PATENT APPLICATION

<§) Date of filing: 28.08.85

© Date of publication of application: © Applicant: Interox Chemicals Limited 11.03.87 Bulletin 87/11 Hanover House 14 Hanover Square London W1R OBE(GB) (w) Designated Contracting States: AT BE CH DE FR GB IT U LU NL SE @ Inventor: Broome, Andrew David John 5, Smithills Close Birchwood Park Warrington Cheshire, WA3 7LT(GB)

® Representative: Pearce, Timothy et al Patent Department Laporte Industries Limited P.O. Box 2 Moorfield Road Widnes Cheshire WA8 OHE(GB)

(sy . Manufacture of arsenic acid.

@ Manufacture of arsenic acid by oxidation of arsenic trioxide with hydrogen peroxide is potentially hazardous, especially when making concentrated solutions of arsenic acid as sought by the users. The process also can suffer from substantial wasteful decomposition of hydrogen peroxide. The problems arise from or are exacerbated by the nature of the reactants, the exothermic character of the principal and decomposition reactions and the formation of localised hot spots. The problems can be ameliorated by introducing the reagents with agitation into a body of reaction product and maintaining the reaction mixture at a temperature of not more than 70 °C and preferably 45 to 55 °C. By carrying out the process in such a manner, it can accomodate even low grade arsenic rf trioxide as reagent.

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Xerox Copy Centre 0 213 244

Manufacture of Arsenic Acid

The present invention relates to a process for been heated to boiling point by the bubbling the manufacture of arsenic acid and in particular through of live steam.. Subsequently, cold water is the manufacture of an aqueous solution thereof. added to the reaction vessel and the mixture is In aqueous solution, arsenic acid has the for- then slowly heated up to boiling point which is mula H3As04. It is crystallised as a hydrate and can 5 maintained for a subsequent 2-3 hours. The Rus- be progressively dehydrated by the stepwise re- sian Patentees confirmed that such a process was moval of water molecules to form respectively the complex and took a long time, and suffered from meta and pyro arsenic acids. Arsenic acid as such, the disadvantage that the hydrogen peroxide must or in dehydrated form has long been used as a be introduced slowly during which period strict biocide and as a component of a class of timber 70 temperature control was necessary, since distur- preservative products. bance of the temperature conditions could lead to Hitherto, methods and proposed methods for an explosion. In view of the disadvantages in that the industrial production of arsenic acid have in process, the Russian Patentees employed, instead, many instances included the oxidation of arsenious hydrogen peroxide diluted to a concentration of 10- oxide which has the formula As*O3 and which is 75 20% concentration and they alleged that this could known also by a variety of other names such as be employed at between 110-150% of the white arsenic, crude arsenic, arsenous oxide or, stoichiometric amount to oxidise the arsenic tri- confusingly, simply as "arsenic". Although a range oxide. However, even when they used pharmaceu- of oxidising agents has been proposed, manufac- tical grade reagents, i.e. reagents free from sub- turing activity has centred upon the use of nitric 20 stantial amount of impurities that would catalytically acid as oxidant, together with some appropriately decompose hydrogen peroxide, Example 2 needed selected catalyst. Such as process has continued 170% of the stoichiometric amount of hydrogen to find favour despite the known disadvantage that peroxide and even so produced a final solution the reaction produces substantial volumes of gas- concentration of only about 30% arsenic acid. Such eous nitrogen oxides, leading to foaming and boil 25 a product is rejected by the use industry which over problems and which need a large plant to demands arsenic acid concentrations in the region scrub them from the effluent and be oxidised to of at least 65% and preferably 75-85% w/w for reform nitric acid. Also the process suffers from the which the use of a more concentrated hydrogen of problem temperature control, and that such, peroxide reagent rather than a diluted reagent problems are exacerbated by the enforced move 30 would be required. In other words, in order to towards the use of lower grade arsenic trioxide satisfy the requirements of the industry, in terms of starting material. final arsenic acid concentration, any proposed hy- Although other methods have been proposed, drogen peroxide process must simultaneously these too suffer from disadvantages. Thus, for ex- avoid the dilution taught by the Russian Patentees ample, in GB-A-2113194, the nitric acid method is 35 and also ameliorate their decomposition problems modified by using oxygen as the oxidant and nitric whilst overcoming the control problems inherent in acid as a catalyst only in conjunction with iodide, in using concentrated hydrogen peroxide which the a pressurised reactor, but it would appear from the Patentees identify in their summary of the prior art. text that the amount of nitric acid present is critical, In the course of investigating the reaction be- in that if too small an amount is present, the 40 tween arsenic trioxide and hydrogen peroxide, the reaction stops prematurely whereas if excess is inventor of the present invention carried out a se- the present continued regeneration of iodide is ries of trials. Theoretical calculations showed that a suppressed, which would therefore prevent the re- concentrated arsenic acid product demanded the action cycle to proceed in the desired manner. In use of concentrated hydrogen peroxide as reagent, view of the fact also that nitric acid can be con- 45 especially if a significant proportion would be lost sumed by impurities in the starting material, it will by decomposition. He determined that addition of be recognised that operation of the process can commercially available arsenic trioxide to a body of be readily disrupted. a concentrated hydrogen peroxide would represent One further oxidising agent that has been pro- a hazardous process especially on a full size man- posed is hydrogen peroxide, in SU-A-510430. In 50 ufacturing scale since the presence from the start this specification, the Russian Patentees describe of the entire amount of concentrated hydrogen two methods of operation using hydrogen peroxide. peroxide would not only maximise the loss due to In the first method, 30-35% hydrogen peroxide is decomposition of the hydrogen peroxide induced added dropwise or in a fine stream at 60-70°C into by impurities from the arsenic trioxide introduced, of a slurry arsenic trioxide and water which has but also would thus maximise the risk of foaming, 0 213 244 ejection of product from the reaction vessel or at that the reaction exotherm could be readily con- worst promote an explosion. On the other hand, trolled, and losses of reagent by decomposition when concentrated hydrogen peroxide is added to. kept down to acceptable levels, even with impure the arsenic trioxide, there are once again significant arsenic trioxide starting material. safety problems, in that, at least initially, the mix- 5 Advantageously, the process of the present ture forms an extremely dense slurry that is very invention can accommodate the use of concen- hard to mix, with the result that the exothermic trated hydrogen peroxide, that is to say at least nature of the oxidation reaction coupled with very 35% w/w H2O2l preferably at least 45% w/w H2O2 poor heat transfer on a manufacturing scale pro- and normally up to 75% w/w H2O2. Even allowing motes the formation of localised hot spots in which 70 for some loss of hydrogen peroxide due to rapid decomposition of hydrogen peroxide is ob- impurity-induced decomposition, it is therefore pos- servable together with steam evolution and the sible to produce, directly and in relative safety, an inherent dangers associated with such peroxidic arsenic acid solution having a concentration of at decomposition. On a manufacturing scale, the least 65% and preferably in the region of 75-85% problems are exceedingly difficult to control, other 75 by the selection of appropriately concentrated hy- than by very slow addition of the reagent. Such drogen peroxide. In other words, the process is findings thus confirm the analysis of the prior art readily capable of producing a product having a made by the Russian Patentees. concentration desired by the use industry. It is Accordingly, it is an object of the present in- especially suitable to employ from 50% w/w H2O2 vention to provide a process which is capable of 20 to 70% w/w H2Oj in that such solutions are com- making concentrated arsenic acid solution by oxi- mercially available and readily transportable, as dation of arsenic trioxide with hydrogen peroxide, well as producing a high concentration product. but avoiding or ameliorating the aforementioned The hydrogen peroxide solution can contain manufacturing difficulties. one or more stabilisers that are effective in an According to the present invention there is 25 acidic medium, such as pyrophosphates, and/or provided a process for the manufacture of arsenic orthophosphates, and/or organic phosphonates in- acid by reaction between arsenic trioxide and hy- cluding hydroxy alkyl diphosphonates and amino drogen peroxide in which the reagents are intro- methylene phosphonates and/or wholly organic duced with agitation into a body of fluid comprising stabilisers such as dipicolinic acid and hydrox- an aqueous solution of arsenic acid, and maintain- 30 yquinoiines and phenols, optionally substituted ar- ing the temperature of the reaction mixture at a ound the aromatic nucleus by non-interfering sub- temperature below 70°C. By introducing the stituents. It will be recognised that the value of reagent in such a manner, it is possible to amelio- including such stabilisers increases as the purity of rate or overcome the disadvantages inherent in the the arsenic trioxide starting material diminishes and prior art methods of using hydrogen peroxide and 35 its particle size increases. especially the disadvantages and danger that may It is desirable to add at least a stoichiometric occur when one reagent is brought directly into amount of hydrogen peroxide, based on the ar- contact with the other reagent. senic trioxide, namely at least 2 moles per mole of It is convenient to consider the present inven- arsenic trioxide of formula As,O3. In practice, a tion in the context of batch reactions, one espe- 40 minor proportion of the hydrogen peroxide is often cially convenient method comprising a cycle in consumed by side reactions or decomposition and which one batch-worth of both reagents is intro- accordingly it is preferable to employ a suitable duced progressively into the body of arsenic acid excess amount of hydrogen peroxide in each and permitted to react for at least a minimum batch. Purity is one of the factors influencing the reaction period and then the corresponding volume 45 selection of the amount of hydrogen peroxide, and of product is withdrawn from the augmented body a second factor is the particle size of the arsenic of aqueous arsenic acid solution. It is preferable for trioxide. It is normal for the amount of hydrogen the body to comprise at least one batch-worth of peroxide to be selected in the range of 105% of reagents and in practice, on a manufacturing scale the stoichiometric amount to 200%, in the context conveniently comprises at least 5 and often from so of an As2O3 purity ranging from above 99% down 10-15 batches-worth of product. Conveniently this to 92%. By way of example if the starting material can be expressed in terms of the moles of arsenic is rather coarse then, for high purity As2O3 starting present in the body of arsenic acid solution and material, it is advisable to employ from 105% to moles introduced per batch of arsenic trioxide. By 115% of the stoichiometric amount; for the inter- carrying out the process in this way, it was found 55 mediate purity material around 98% purity, it is convenient often to use from 115-130% of the stoichiometric amount of hydrogen peroxide; for low grade starting material of e.g. 96-97% purity it 0 213 244

is usually desirable to select in the range of 120- temperature is controlled to at least 40°C. It will be 150% of the stoichiometric amount, and for really recognised that the provision of the arsenic acid low grade material, such as below 95% purity, it is body of fluid assists in the maintenance of a con- usual to employ at least 125%, and typically up to trolled reaction temperature in that it provides a 175%. However, when more finely ground starting 5 larger volume for the dispersal of the heat of reac- material is used, less hydrogen peroxide is wasted tion, thereby, at the same time, diluting any inher- in side reactions or decomposition with a result that ent temperature rise and presenting an augmented all the nearly arsenic present can be oxidised into volume for external cooling means as well as pro- solution using no more than 125% of the viding a more fluid medium and thus reducing heat stoichiometric amount for any material having a io transfer problems. By so doing, it is able to obviate purity of 90% arsenic or over, in practice often or at least ameliorate problems of localised tem- from 105 to 125%. Under-addition of hydrogen perature build-up to above the SADT (self accel- peroxide can result in particulate arsenious oxide erating decomposition temperature). In practice, the in remaining suspension at the end of the reaction, reaction mixture temperature is maintained at up to and to avoid build-up of solids the residual amount 75 60°C, and especially at around 45 to 55°C, in should be filtered off or alternatively be taken into order to balance the twin and opposed criteria of account in calculating the amount of hydrogen per- increasing reaction rate versus reduced safety mar- oxide to add in the subsequent batch. gin. Hence increased potential throughput is bal- It will further be recognised that use of an anced against an increased likelihood of the reac- increasing excess amount of hydrogen peroxide 20 tion passing out of control when a higher design introduced at a given concentration translates to a temperature of the reaction is permitted. In prac- dilution of the resultant arsenic acid product. Such tice, it will be recognised that the temperature is dilution can be offset, if desired, by employing a often controlled by balancing the cooling with the correspondingly higher concentration of hydrogen rate of introduction of, in particular, the aqueous peroxide. 25 hydrogen peroxide solution. It is most convenient for the arsenic trioxide to It is a further advantageous feature of the probe introduced in powder form. To at least some cess described herein that even though the reac- extent there is a correlation between the efficiency tion mixture is preferably maintained at a tempera- of use of hydrogen peroxide for oxidising the ar- ture of at least 35°C, it is normally possible to senic trioxide and the particle size of the latter, 30 commence the reaction at markedly lower tempera- smaller particles being more favoured. This can tures, without the provision of external heat. Thus, manifest itself as an overall reduced hydrogen per- for example, even where the body of arsenic acid oxide requirement or in improved separation of solution initially has a temperature of 0°C, introduc- arsenic from the impurities in the starting material. tion of the reagents can be made and within a It is desirable for the average particle size to be 35 single batch, the reaction temperature can normally below 500 micrometres, and in many instances will restore itself to the preferred temperature condi- be at least 40 micrometres. The starting materials tions. in many instances have such low particle sizes but In practice, the reaction is conducted by in- if not they can be achieved by conventional grind- troducing the reagents simultaneously or alter- ing equipment. It will be recognised that very small 40 natively, or aliquots of the solid reagent and con- particles of e.g. below 50 micrometers can form tinuous addition of the aqueous hydrogen peroxide agglomerates e.g. up to 200 micrometers in size. into the body of arsenic acid solution until the Such agglomerates can also be used very readily desired amount of each has been introduced. It in the instant process. convenient for the reagents to be introduced over a One important feature of the present invention 45 period of from 1 to 5 hours and the overall reaction is the temperature which is maintained in the reac- period to be selected within the range of from 2 to tion vessel during the introduction of the reagents 10 hours, including the period during which the and the subsequent reaction. The reaction itself is reactants are introduced. It will be recognised that, exothermic and thus it has been found in practice to at least a certain extent, the reaction period is that a temperature of above ambient can be main- so interlinked with the reaction temperature and that tained throughout the reaction period without in- should the reaction be conducted at a temperature troduction of external heat, and indeed the mixture from ambient to 35°C, proportionately longer is typically subjected to external cooling throughout periods of introduction and reaction time should be the reaction period. The rate of the reaction, is employed. Alternatively or by way of supplement, a however, somewhat sensitive to temperature, and it 55 sample of the reaction mixture can be analysed at is preferable therefore to maintain a temperature of intervals for residual hydrogen peroxide concentra- at least 35 °C in order to maintain an acceptable tion and the product withdrawn when substantially rate of reaction and in many embodiments, the no hydrogen peroxide remain. 0 213 244

In some valued embodiments, especially when in both instances the body of arsenic acid product all or most of the particles of the- arsenic trioxide and reactant being mixed to such an extent that starting material are below 500 micrometers, the when it comes into contact with the second starting process is operated according .to the following material, no such localised concentration of the first combination of process conditions. A batch of par- 5 starting material have been permitted to remain. ticulate arsenic trioxide containing at least 92% by One particularly convenient method for effecting weight As203 and from 105 to 125% of the such a separation comprises the introduction of the stoichiometric amount of hydrogen peroxide as an two reactant s into separated zones of the body of aqueous solution containing 45 to 75% w/w H202 fluid, such as into separate vessels with the body are introduced gradually into a body of aqueous w of reaction mixture circulating between the two of arsenic acid containing from 5 to 15 moles As per them. In practice, such a circulating reactor system mole As in said batch, thereby forming a reaction normally requires fluid to be pumped from one mixture, the reaction mixture is agitated and main- vessel to the other from which it is permitted to tained at a temperature in the ranged of 40 to 60oC flow under gravity such as over a weir back to the for a period of at least 2 hours after introduction of 75 first vessel and the pumping itself can provide the reagents commences, and thereafter a fraction sufficient agitation in respect of the vessel into of the mixture is withdrawn as product. Within that which the aqueous hydrogen peroxide is added. In combination of conditions, the earlier identified nar- the other reaction vessel, it is preferable to incor- rower preferred ranges for any or all of those porate a mechanical agitator that is of sufficient conditions can naturally be used. 20 power and is adapted to blend the surface layer of In order to assist the reaction, and particularly solid particles within the main body of the fluid in in conjunction with the processing of lower grade that vessel. starting material, it can be advantageous to intro- The fluid product typically contains a residual duce a catalytic amount of an iodide, particularly amount of solids, part of which represent unreacted potassium iodide into the reaction mixture, typically 25 arsenic trioxide and part impurities. It is most con- in an amount of up to 0.2% w/w and especially in venient for such solids to be filtered off. Although an amount from 0.01 to 0.10% w/w. In addition, it is all the body of arsenic acid could be filtered after desirable to promote wetting of the arsenic trioxide the production of each batch, it is generally prefer- on introduction into the reaction mixture, by includ- able to filter merely the amount of product with- ing, for example, a small amount of a wetting 30 drawn. It has been observed that the small amount agent, typically bentonite in a total amount of up to of solid separated at this stage tends to include 0.1% w/w and often from 0.01 to 0.03% w/w on the rather large particles compared with the average mixture. By promoting the wetting of the arsenic particle size of the starting material. Analysis of the trioxide, the period is reduced during which it floats solids has shown that they tend to have a lower upon the surface of the reaction mixture where it 35 arsenic content than that of the starting material, can form localised spots from which heat dissipa- indicating that preferential dissolution of the arsenic tion is slow. has occured. Advantageously, such preferential dis- One aspect of the invention of practical impor- solution seems, on present indications, to be no- tance when the reaction is carried out on a large ticeable with the relatively impure starting materials scale is the thorough mixing of the reagents with 40 i.e. those in which the potential problem of impuri- the body of arsenic acid into which are introduced. ties is greatest. In view of the foregoing, it can be It will be recognised that concentrated arsenic acid advantageous not to recycle the separated solids, solutions, for example those containing over 70% although where the impurity content of the product w/w arsenic acid are dense, viscous fluids. Thus, would still fall within customer-tolerated limits there is a tendency especially for the arsenic tri- 45 solids recycling can result in some further recovery oxide starting material to float, at least initially, of arsenic. because conventionally it is in the form of a pow- It is preferable for the reaction vessel to be der. When it floats it remain as a crust of small operated at slightly sub-ambient pressure with particles on the surface of the body of the reaction vented gases being passed through a scrubbing mixture. It is recognised that the benefit of the so system designed to remove all arsenic oxide par- instant invention could be dissipated if the reagents ticles and arsenic acid solution entrained in the were permitted to come into contact primarily as gases. Since the primary oxidant hydrogen perox- surface films or if hydrogen peroxide, for example, ide in the present invention is introduced in liquid is introduced onto a surface comprising principally form rather than as oxygen gas and does not a crust of unreacted arsenic trioxide starting ma- 55 generate substantial gaseous reaction products like terial. In consequence, it is highly advantageous for the introduction of the two starting materials to be separated, either by location or sequential addition, 0 213 244 10 nitrogen oxides, the scrubbing system need only peroxide in a continuous stream. Bentonite (3g, be small scale in compounds with plants of com- 0.02% of the total arsenic trioxide) was introduced parable manufacturing capacity employing respec- into the reactor together with the first aliquot of tively air or nitric acid as oxidant. arsenic trioxide. Having described the invention in general During the course of introduction of approxi- terms, specific embodiments will now be described mately the first 10% of the reactants, the reaction more fully by way of example only. Attention is vessel was agitated, but cooling water was not drawn to the poisonous nature of both arsenic being circulated through the reactor jacket. During trioxide and arsenic acid to humans and the need the period the temperature of the reaction mixture therefore for appropriate safety precautions to be 10 rose from ambient temperature to about 40 °C. Dur- adopted in order to avoid contact with them. ing the remainder of the reaction period, the cooling water was circulated at such a rate that the temperature in the vessel was maintained at 40°C Examples 1-7 +/- 3°C. The reaction was allowed to continue at 75 40 °C for a further hour after all the reagents had Each of Examples 1-7 was carried out in a been introduced and then the reactor contents jacketted 60 litre stainless steel vessel fitted with were discharged and the total residual solids con- an agitator and temperature probe and having an tent of the batch determined. From this determina- offtake for product at its base. The vessel was tion, the extent to which the arsenic had been cooled by passing mains supply water through the 20 converted to arsenic acid was determined and, jacket. expressed as a percentage in the following Table In each Example, 15 litres of an aqueous ar- under the term "reaction efficiency". Naturally, al- senic acid solution having a concentration of ap- lowance was made for any other solids present proximately 75 to 80% w/w arsenic acid was intro- such as the added bentonite and any solids introduced into the reaction vessel, forming the body of 25 duced with the original body of arsenic acid solu- product into which the reactants would subsequent- tion. ly be introduced. In Example 1 the body comprised In a modification of Example 1 in Examples 3 laboratory grade arsenic acid and in the subse- to 7 inclusive a catalyst,' potassium iodide (50g, quent Examples product from earlier Examples was 0.1% w/w of the arsenic trioxide) was introduced used instead. 30 together with the first aliquot of arsenic trioxide. In Initially the body of arsenic acid has a tem- a further modification in Example 7 only, the reac- perature of about 16°C, namely ambient tempera- tion temperature was permitted to rise to 50°C ture. Arsenic trioxide (15Kg) and a supra- whereupon it was subsequently controlled to within stoichiometric amount of hydrogen peroxide (50% +/- 3°C by adjustment in the rate of flow of w/w, commercially available from Interox Chemicals 35 cooling water. Limited) indicated in the following Table 1 were Periodically, samples of the liquor in the reac- then each introduced into the reaction vessel over tion vessel were taken and analysed for residual a period of 3 hours, the arsenic trioxide being peroxide content. The reaction conditions and re- introduced in 30 equal aliquots and the hyrogen sults are summarised in Table 1 below. 40

55 11 0 213 244 12

Table 1 ' Example AS2O3 H2O2 Product Analysis Reaction No. purity addition Total H3ASO4 efficiency % % solids % w/w %' gl"1 1 99.9 110 3.7 78.5 99.3 2 99.9 110 4.8 81.0 99.1 3 94.9 150 1.0 74.8 99.8 4 94.9 120 5.3 76.6 99.0 5 94.9 135 4.6 75.1 99.1 6 99.1 120 6.8 80.0 98.7 7 99.1 120 2.6 84.0 99.5

From Table 1 above, it will be seen that using was introduced into a round bottom flask equipped the method of the instant invention, it was possible with a stirrer and then over a period of 3 hours 50 to readily obtain an arsenic acid concentration of gms of arsenic trioxide was introduced slowly to- around 75% or higher even from starting material 25 gether with 0.01 g bentonite. The temperature was as impure as below 95% arsenic trioxide. Thus, it maintained at 40°C. After all the arsenic trioxide will be observed that with all grades, a reaction had been introduced, the reactants were allowed to efficiency of over 99% was attainable and from a react for a further 1 hour at the reaction tempera- consideration of the total solids content it can also ture. The product was analysed as in the Exam- be seen that not all of the impurities went into 30 pies. At the end of the reaction it was found that solution. By a comparison of Examples 1 and 2 only 96.5% oxidation to arsenic acid had occurred, with subsequent Examples 3 to 5 and 6/7, it will be with a peroxide residual equivalent to 3% of the recognised that there is a broad correlation be- stoichiometric amount. The reactants were modi- tween purity of the arsenic trioxide starting material fied by also adding potassium iodide as catalyst. In and the amount of hydrogen peroxide required for 35 the presence of 0.1% w/w and 0.5% w/w potassium virtually complete oxidation. By comparison, it will iodide, the extend of oxidation was increased to be recalled that Example 2 of SU-A-510430 with respectively 97.8% and 96.9% with peroxide re- pharmaceutical grade As2O3 needed 175% of the siduals of 3% of the stoichiometric amount and stoichiometric amount of hydrogen peroxide and 0.3% of the stoichiometric amount. Increasing the even then could produce a concentration of only ^ reaction temperature to 50 °C also produced a 29%. It will also be observed by comparison be- 96.9% oxidation but with no residual peroxide. It is tween Examples 6 and 7 that increasing the reac- readily apparent that this technique was substan- tion temperature from 40 to 50° enhanced both the tially more wasteful in terms of peroxide decompo- reaction efficiency and reduced the total solids sition than the technique according to the present content in the product liquor. ^ invention. By way of comparison, further experiments were conducted into the reaction between hydrogen peroxide and arsenic trioxide in which the Examples 8 to 13 reactants were brought directly into contact with each other, and not indirectly through separate 50 Each of these small scale Examples was car- introduction into a body of the reaction product. ried out using a very low grade arsenic trioxide These further experiments were conducted using starting material containing 70.15% arsenic, cal- an intermediate grade of arsenic trioxide containing culated as the metal, equivalent to 92.6% as AsjOj, 97.2% w/w As2Oj. In view of the increased hazard (50g) and 50% w/w aqueous hydrogen peroxide at when employing such a technique, they were car- S5 150% or 175% of the stoichiometric amount. In ried out on a smaller scale, as follows. A predeter- mined amount of hydrogen peroxide, 150% of the stoichiometric amount and at 35% w/w solution, 13 0 213 244 14

Examples 8 and 9, the arsenic trioxide had been Kl and 0.02% w/w bentonite was introduced with previously ground to a particle size of below 710 the first aliquot of arsenic trioxide. The temperature micrometres and in Examples 10 to 13 to below of the reaction mixture was held at 50 °C during the 425 micrometres. introduction but during the subsequent period of an In each of these Examples a body of arsenic hour cooling to around 40 °C was observed. The acid solution (82. 7% w/w, 105g) was introduced product was then filtered, the solid residue an- into a glass vessel equipped with a stirrer and a alysed and the % of arsenic that had been oxidised baffle, separating the point of introduction of the was calculated. two reagents subsequently. During the next three The results are summarised in Table 2, the hours, arsenic trioxide was introduced with stirring w arsenic content in the residue being calculated as at approximately evenly spaced intervals in aliquots ASjOj and the impurity content being the balance of about 0.5g, and the hydrogen peroxide was and thus including any undissolved K and ben- pumped in a constant rate. Additionally, 0.1% w/w tonite.

Table 2 Example H2O2 Solid Residue % of No. Addition Total Arsenic Impurity arsenic % weight Content Content oxidised g g g 8 175 6.6 3.4 3.2 92.9 ' 9 150 5.9 3.4 2.5 92.9 10 175 4.4 1.7 2.7 96.4 11 150 4.0 1.2 2.8 97.4 12 125 3.4 0.8 2.6 97.7 13 110 3.8 0.9 2.9 97.6

From Table 2, it can be seen clearly that the 3. A process according to claim 1 or 2 charac- solid residue contains a much higher proportion of terised in that 35 the arsenic trioxide has a purity of at the impurities than did the starting material indicat- least 90% w/w. ing preferential solubilisation of the arsenic. Sec- 4. A process according to any preceding claim ondly, it will be noted that the smaller particle size characterised in that the body of fluid into which of Examples 10 to 13 led to improved arsenic the reagents are introduced comprises at least half oxidation whilst leaving a similar amount of impuri- the volume of 40 mixture after all the reagents have ties as solid residue. been introduced. 5. A process according to any preceding claim characterised in that the reaction mixture is con- Claims trolled to a temperature of 35°C to 60°C. 6. A 45 process according to claim 5 charac- 1. A process for the manufacture of arsenic terised in that the reaction mixture is controlled to a acid by reaction between paniculate arsenic tri- temperature of 45°C to 50°C. oxide and aqueous hydrogen peroxide characteris- 7. A process according to any preceding claim ed in that the reagents are introduced with agitation characterised in that the amount of hydrogen per- into a body of fluid comprising an solution oxide introduced aqueous 50 is selected within the range of of arsenic acid, thereby forming a reaction mixture 105 to 200% of the stoichiometric amount. the reaction mixture is maintained at a temperature 8. A process according to claim 7 charac- below 70 °C, and thereafter a portion of the mixture terised in that the amount of hydrogen peroxide is withdrawn as product. introduced is selected inversely to the purity of the 2. A process according to claim 1 charac- arsenic trioxide. 55 terised in that hydrogen peroxide is introduced as an aqueous solution containing 35% of 75% w/w hydrogen peroxide. 15 0 213 244 16

9. A process according to any preceding claim 11. A process according to any preceding characterised in that the reaction period lasts 2 to claim characterised in that the two reagents are 10 hours after introduction of the reagents com- introduced simultaneously into two zones of the mences. body of fluid which are then brought into contact. 10. A process according to~ any preceding 12. A process according to claim 11 charac- claim characterised in that a batch of particulate terised in that the body of fluid is recycled between arsenic trioxide containing at least 92% by weight the zones. As,O, and from 105 to 125% of the stoichiometric 13. A process according to any preceding amount of hydrogen peroxide as an aqueous solu- claim employing up to 0.5% w/w based on the tion containing 45 to 75% w/w H2O2 are introduced 70 arsenic trioxide starting material of an oxidation gradually into a body of aqueous arsenic acid catalyst preferably potassium iodide. containing from 5 to 15 moles As per mole As in 14. A process according to any preceding said batch, thereby forming a reaction mixture, the claim characterised in that up to 0.5% w/w based reaction mixture is agitated and maintained at a on the arsenic trioxide starting material of wetting temperature in the range of 40 to 60° C for a period 75 agent preferably bentonite is introduced therewith. of at least 2 hours after introduction of the reagents commences, and thereafter a fraction of the mixture is withdrawn as product.

55 J European Patent Application number EUROPEAN SEARCH REPORT Office EP 85 30 6083

DOCUMENTS CONSIDERED TO BE RELEVANT Citation of document with Category indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION (Int. CM)

C 01 G 28/00 t,D SU-A- 510 430 (N.L. GUDKOV et al.)

^D GB-A-2 113 194 (RENTOKIL LTD.)

TECHNICAL FIELDS SEARCHED (Int. CM )

C 01 G 28/00

The present search report has been drawn up for all claims Place of search Date of completion of the search Examiner BERLIN 14-04-1986 CLEMENT J.P.

CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date § Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document