NZ248287A - Production of arsenic acid by oxidation of arsenic trioxide with hydrogen peroxide and its use in the production of copper chrome arsenate - Google Patents

Description

24 8 2 Patents Form # 5 NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION TTTT F.: Manufacture of Arsenic Acid , I,: John Ayrton Branch Address: State Highway 12, Taheke, Kcukohe, New Zealand Nationality: New Zealand hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: V PF05.JWP if .

I J' " FEE CODE -1050 :'V-J 24 82 8 FIELD OF THE INVENTION 5 This invention relates to a process for the production of arsenic acid.

BACKGROUND OF THE INVENTION Hitherto, methods and proposed methods for the industrial production of arsenic acid 10 have in many instances included the oxidation of arsenic trioxide which has the formula As203. This substance is also known by a variety of other names such as arsenious oxide, white arsenic, crude arsenic, arsenous oxide or simply as "arsenic". As203 will be referred to herein as arsenic trioxide.

The methods which have hitherto been used (or proposed to be used) industrially to make arsenic acid are well summarised in the specification of US patent #4891207 and its equivalent New Zealand patent #210496 and the following discussion of the background to the present invention is taken largely from the specification which is substantially common to these two patents. In that specification it is noted that in 20 aqueous solution, arsenic acid has the formula HjAsO^ It is crystallised as a hydrate and can be progressively dehydrated by the stepwise removal of water molecules to form respectively the meta and pyro arsenic acids. Arsenic acid as such, or in dehydrated form, has long been used as a biocide and as a component of a class of timber preservative products.

Although a range of oxidising agents have been proposed for reducing arsenic trioxide to make arsenic acid, manufacturing activity has centered upon the use of nitric acid as oxidant, together with some appropriately selected catalyst. The latter process has continued to find favour despite the known disadvantage that the reaction produces 30 substantial volumes of gaseous nitrogen oxides, leading to foaming and boil over problems, and which need a large plant to scrub them from the effluent and be oxidised to reform nitric acid. Also, in the use of the process, temperature control is a problem. These problems are exacerbated by the enforced move towards the use of lower grade arsenic trioxide starting material.

Although other methods have been proposed, these too suffer from disadvantages. 6C06NZCS.794\iv 2* 82 8 7 Thus, for example, in GB-A-2113194, the nitric acid method is modified by using oxygen as the oxidant and nitric acid as a catalyst only in conjunction with iodide, in a pressurised reactor, but it would appear from the text that the amount of nitric acid present is critical, in that if too small an amount is present, the reaction stops prematurely whereas if excess is present the continued regeneration of iodide is suppressed, which would therefore prevent the reaction cycle to proceed in the desired manner. In view of the fact also that nitric acid can be consumed by impurities in the starting material, it will be recognised that operation of the process can readily be disrupted.

There are further methods disclosed in the prior art which are of relevance to the present invention. The first is a method disclosed in the specification of Russian patent #SU-A-510430 which, incidentally, is also discussed in the preamble to US patent #4891207. In the Russian specification hydrogen peroxide, in a concentration of 30-35% in water, is added to a slurry of arsenic trioxide in water at boiling point. The temperature of the is 60-70° C. Subsequently, cold water is added to the reaction vessel and the mixture is then slowly heated up to boiling point and maintained at that temperature for 2-3 hours. In a modification of the above process disclosed in the same Russian patent, the hydrogen peroxide was diluted to 10-20% in water. However, it is stated in US patent #4891207 that even if an amount of equal to 170% of the stoichiometric amount is used, the concentration of arsenic acid in the final solution is only about 30%. According to a further statement made in the preamble to the specification of US patent #4891207, industry demands arsenic acid concentrations of at least 65% and that the product produced by the process described in the Russian patent would therefore not be acceptable.

Another method of relevance to the present invention is that disclosed and claimed in US patent #4891207 as the invention itself. In this method, particulate arsenic trioxide without dilution in water and aqueous ^lf>2 *n a concentration of at least 35% w/w are introduced separately into an aqueous solution of arsenic acid in a concentration of at least 65% w/w. The reaction is carried out at a controlled temperature of 35-70° C. The method produces an additional amount of arsenic acid in a concentration of 65% or more. The reaction is said to take between 2 and 10 hours and an amount of hydrogen peroxide of not more than 125% of the stoichiometric amount is said normally to be used. A catalyst such as potassium iodide is used at the start of the process as is a 6606NZCS.794\iv 248 28 ? wetting agent such as bentonite.

Also relevant to the present invention is the disclosure in US patent #4891207 of a 5 laboratory experiment in which arsenic trioxide was introduced slowly into a flask containing 35% w/w hydrogen peroxide, the temperature being maintained at 40° C. In the applicant's experience this process has several disadvantages on an industrial scale. The arsenic trioxide powder must be added slowly and has a tendency to float on the surface of the H202 causing localised hot-spots which lead to decomposition of the 10 H202 as noted in the US patent.

It is well known that close control must be kept of all processes for making arsenic acid since they are exothermic and hence subject to thermal runaway. It is an object of the present invention to provide a method for making arsenic acid which is susceptible to 15 such control and at the same time has been found, surprisingly, to result in a solution having an acceptably high arsenic acid concentration.

SUMMARY OF THE INVENTION According to the invention, there is provided a process for the production of aqueous arsenic acid including the steps of: (i) providing in a mixing vessel a body of liquid consisting essentially of arsenic trioxide and water; (ii) while mixing the liquid, adding hydrogen peroxide to the liquid (a) at a rate which is adjusted so that the temperature of the liquid is kept between 35° and 75° C and (b) in an amount equal to between 100% and 125% of the stoichiometric amount of arsenic trioxide used in the production of the batch, the temperature of the hydrogen peroxide when it is added to the liquid being substantially less than that at which it is subject to thermal decomposition.

It is known that self accelerating thermal decomposition of the hydrogen peroxide begins to occur when the temperature of the liquid rises above about 50° C. In the 35 processes disclosed in the prior art referred to herein, the rate of decomposition above 55 ° C is such that the processes are increasingly difficult to control A * 6606NZCL.995/JW/iv '27 SEP 7995 1 dangerous, especially on an industrial scale. Furthermore, it is apparent that, with increasing temperature, the processes are increasingly wasteful of H^.

Similarly, it has been observed by the applicant that, when the temperature of the liquid drops below about 35° C, the rate of reaction between the reactants becomes so slow that, not only would the process be uneconomic on an industrial scale but there is a substantially increased possibility of a build up of hydrogen peroxide in the liquid. Again this is dangerous, especially on an industrial scale.

The HjOj is preferably at ambient temperature when added and in any case should not be heated to a temperature at which it decomposes before it is added to the mixing vessel. If the concentration of the added to the liquid in the mixing vessel is high, the amount of water in the liquid can be increased, depending on the required concentration of the arsenic acid end product. On the other hand, if the concentration of the H202 added to the liquid in the mixing vessel is low, the amount of water in the liquid can be decreased.

According to one aspect of the invention, the hydrogen peroxide is introduced into the liquid below the upper surface of the liquid.

In one form of the invention, an amount of 100% to 125%, and preferably 103% to 110%, of the stoichiometric amount of hydrogen peroxide is added to the liquid. This should produce a solution in which the arsenic acid is at a satisfactory concentration, particularly for subsequent use in the preparation of a copper chrome arsenate solution.

The Hj02 will in most cases be added in a period of 3.5 to 5 hours and the process can still be kept under adequate control. The time taken is a function of the volume of the liquid in the mixing vessel and the efficiency of the cooling system.

In one aspect of the invention the liquid in the mixing vessel is heated to between about 30° and 50° C before addition of the hydrogen peroxide commences. If the starting temperature of the liquid is too low there is an increased risk of a build up of in the liquid. Clearly, on the orher hand, it is undesirable that the starting temperature of the liquid should exceed the upper temperature to which the controls are set 6606NZCS.794UV 8 2 87 In one form of the invention the arsenic trioxide is present in the body of liquid in a proportion of between about 1 and 2 parts by weight to 3 parts water present before introduction of the hydrogen peroxide commences. According to one aspect of the invention, further amounts of arsenic trioxide are added to the liquid from time to time so that, after about an hour all of the arsenic trioxide has been added and this makes up between 150% and 300% by weight of the original amount of water in the vessel.

Further according to the invention, a copper chrome arsenate solution is prepared by adding Cr03 and CuO in suitable form to an aqueous solution of arsenic acid solution prepared according to the present invention. The Ci03 is preferably in flake form and the CuO is preferably in powder form. The quantity of Cr03 and CuO to be added will be dictated by the specification for C.C. A preparations put out by the regulatory authorities of the country in which the preparation is to be used or sold.

The invention is further discussed with reference to the following examples and the apparatus shown in Figure 1 of accompanying drawing.

In a practical implementation of the invention described herein 50 kg of arsenic trioxide can be oxidised over four hours. The process can be carried out using the equipment shown in the drawing. This comprises a stainless steel tank 10 which is 0.5m diameter and 1.1m height and having a jacket 12 around the outside of the tank through which coolant water is pumped. The tank has a capacity of 200 litres. The tank is equipped with an agitator/mixer 14 having mixing blades 16 of stainless steel at its lower end and a pair of flat stirring arms 17 above the mixing blades. At the start of preparation of a batch the arms are located above the level of the liquid in the tank. They have the effect of disturbing the build-up of foam in the liquid. The shaft has an operating speed of 34 rpm. Two coolant re-circulating pumps (not shown) are provided. The coolant in the present case is water but other suitable coolants may be used.

The apparatus for introducing HjOj into the tank 10 comprises a header tank 18 from which the is gravity fed into an intermediate tank 20. A float valve 22 senses the level of HjOj in the intermediate tank and cuts off the flow thereof from the header tank when the reaches a predetermined level. From the intermediate tank the F^Oj flows under gravity into the mixing tank 10 through a manually controlled valve 24. In the present case a typical rate of flow of the E^Oj into the tank 10 is about 170 gm per 6606N2CS.794\iv 24 8 2 87 minute. Although the could be pumped into the tank it is preferred that the should gravitate into the tank.

In the present example a temperature controller/sensor is mounted in the liquid. This monitors the temperature and the controller opens or closes a solenoid valve 25, thereby controlling the flow of into the tank 10. In addition the temperature of the liquid is monitored manually, preferably by a bimetal thermometer although a thermo-couple type thermometer could be used.

To begin the process 30 kg of water was introduced into the stainless steel tank and heated over 30 minutes to a temperature of 48°C using an immersion heater. Alternative known methods of heating can be used.

The mixer was started and 20 kg of arsenic trioxide powder (As203) was introduced into the water creating a slurry. The temperature of the slurry dropped to 41#C. The water coolant jacket of the tank was empty.

The small valve 24 was opened and the temperature controller (set at 50°C) was turned on. After a few seconds the solenoid valve opened and hydrogen peroxide (50%) at ambient temperature commenced flowing into the slurry. After four minutes the temperature had increased to 53°C and the temperature controller had functioned to close the solenoid valve thereby shutting off the supply of HjOj.

After one minute circulation of water coolant commenced.

Over the next four hours the flow of hydrogen peroxide into the mixture was regulated primarily by the solenoid valve 25 (automatically) and secondarily by the small valve 24 under the intermediate peroxide tank (manually).

Over the entire period the temperature was recorded at one minute intervals.

During the period of the reaction, the solenoid valve was open for 235 minutes and closed for 11 minutes. The rate of flow controlled by the small valve 24 was adjusted on five occasions. 6606NZCS.794\iv 24 8 2 87 The temperature controller typically terminated the peroxide flow when the temperature reached 51°C and recommenced the peroxide flow when the temperature had dropped The average temperature over the period of the reaction was 49.08°C. For the purposes of the experiment, the period of the reaction is taken to be from the commencement of the peroxide flow until the estimated required amount of peroxide has been used.

In 5 instances the temperature was recorded as being 52'C or more and in 8 instances the temperature was recorded as being 46°C or less.

In 233 instances the temperature was recorded as being between 47°C and 51°C.

A slurry comprising 50 kg of arsenic trioxide in 30 litres of water would be too thick for easy mixing. The remaining 30 kg of arsenic trioxide was therefore introduced (in powder form) as follows: It is important that the H202 is introduced well below the surface of the slurry, otherwise localised hot spots indicating thermal decomposition of the may occur. In the present example, the inlet 26 through which the enters the tank 10 is located at about 30 mm below the level of the slurry at the start of the process. In the course of the process the level of the reactants will rise to about 230 mm above the inlet 26.

To prevent thermal runaway, it is important to put the As203 in as early in the process as possible without allowing a buildup of During the period of the reaction a build up of froth and foam to a depth of 380 mm was to 48#C.

At ASA 31 minutes 46 minutes 61 minutes kg 10 kg 10 kg kg 6606NZCS.794\iv 248287 observed. This build up occurred gradually and reached its peak 195 minutes into the reaction. At 246 minutes the peroxide flow was finished and the foam had subsided. The arsenic acid was stirred for another 60 minutes and left to stand for 12 hours before adding chromic oxide and copper oxide to produce a C.C. A timber preservative. Four hours after peroxide flow had ceased, a spot test indicated the presence of a small amount of peroxide.

The chemicals used were: Water 32 kg Arsenic Trioxide (96.7% purity) 50 kg Hydrogen Peroxide (50%) 40 kg During the period of the reaction when the temperature of the solution dropped below 48°C, the coolant water was manually turned off and turned on when the temperature had increased to 49#C. This occurred on eleven occasions. The total time in which the coolant water was turned off was 64 minutes.

In the above described process, 116% of the stoichiometric amount of hydrogen peroxide was added to the arsenic trioxide mixture. The resulting arsenic acid solution had a specific gravity of 1.56.

Clearly the concentration of hydrogen peroxide added to the liquid in the mixing vessel may be varied. If the concentration of HjOj is too low, the concentration of the arsenic acid produced would also be low and the resulting C.C.A. product might be at a concentration which is less than what is customarily commercially available. It is considered that an concentration of 25% w/w would be about as low as would be commercially viable.

In another example of the preparation of arsenic acid according to the present invention, the chemicals used were Water 30 kg Arsenic Trioxide 50 kg Hydrogen Peroxide (50%) 36.5 kg 6606N2CS,794\iv 24 8 Z 87 The arsenic trioxide used in this example was shown by analysis to contain 2.7% impurities.

The method and apparatus used were substantially the same as those used in the first example. The was introduced into the slurry over a 4 1/2 hour period. The arsenic acid solution formed in the process was stirred for a further half hour. The quantity of used represents a stoichiometric amount of 109.2% calculated on the net amount of available arsenic trioxide in the starting material.

In this example, after the reaction appeared to be complete, an extra 1 kg of was added. It is believed that, with better measuring techniques, the amount of used in the above example could be reduced to 35.0 kg representing 104.8% of the stoichiometric amount The arsenic acid produced in this second example was analysed. The results were: Concentration: 55.3g/100ml = 55.3% Specific gravity: 1.63 @ 20° C.

Arsenic trioxide content: 0.06g/100ml = 0.06% This indicates that 99.91% of the starting material excluding impurities was oxidised.

A copper chrome arsenate slurry was prepared as follows: Arsenic acid solution made up as described in the second example was used as a starting material. The acid solution was left to stand for 16 hours after it was prepared. 4 kg of the solution were removed and the remainder heated to 50° C and 100 kg of Cr03 in flake form was added over 45 minutes. A further 1 kg of water was then added. 30 This was followed by 36.7 kg of black CuO powder, added slowly over 2 hours. The solution was stirred for a further hour. The C.C.A. product was analysed and the results were as follows: 6606NZCS.794\iv 248287 g/100ml Elemental proportion -% Requirements of MP3460:1992 CCA Class l Cu 25.6 Cr 48.2 As 33.4 23.9 44.49 31.2 100.0 23-25% 43-47% 30-32% 107.2 Specific gravity = 2.31 @ 20° C.

This shows that the product is within the specification (Std MP3640/1992) of the New Zealand Timber Preservation Council for a class 1 C.C.A preservative provided that it is diluted since the product is licensed to be sold with an active ingredient content of 650 g/litre.

One of the advantages of the process of the invention is that it is capable of using poor quality arsenic trioxide.

In practice the period of time over which the hydrogen peroxide is added to the arsenic trioxide slurry would vary. Under ideal conditions and using a highly efficient cooling system, the time taken might conceivably be as low as 2 hours. However, it is unlikely that this low rate could be maintained with safety on an industrial scale and the practical minimum rate is more likely to be about 3.5 hours even under good conditions. On the other hand, if the time over which the hydrogen peroxide is added exceeds about 7 hours using apparatus of the size described herein, the process is unlikely to be economically viable.

It is not intended that the scope of a patent granted in pursuance of the application of which this specification forms a part should exclude modifications and/or improvements to the embodiments described and/or illustrated which are within the scope of the invention as defined in the claims or be limited by details of such embodiments further than is necessary to distinguish the invention from the prior art. 6606NZCS.794\iv

Claims (1)

1. 248 28 -12- i. A process for the production of a batch of aqueous arsenic acid including the steps of 5 (i) providing in a mixing vessel a body of liquid consisting essentially of arsenic trioxide and water; (ii) while mixing the liquid, adding hydrogen peroxide to the liquid (a) at a rate which is adjusted so that the temperature of the liquid is kept between 35° and 10 75° C and (b) in an amount equal to between 100% and 125% of the stoichiometric amount of arsenic trioxide used in the production of the batch, the temperature of the hydrogen peroxide when it is added to the liquid being substantially less than that at which it is subject to thermal decomposition. 15 2. A process according to claim 1, wherein the rate at which the hydrogen peroxide is added to the liquid is adjusted so that the temperature is kept between 35° and 55° C. 3 20 A process according to claim 1 or claim 2, in which the hydrogen peroxide is added to the liquid over a period of time of not less than 2 and not more than 7 hours. 4 A process according to any one of claims 1 to 3, in which the hydrogen peroxide is 25 added to the liquid over a period of time of not less than 3.5 and not more than 5 hours. 5 A process according to any one of claims 1 to 4, in which the hydrogen peroxide which is added to the liquid is in a concentration of not less than 25% weight for weight in 30 water. 6 A process according to any one of claims 1 to 5, in which the hydrogen peroxide is -introduced into the liquid below the upper surface of the liquid. 35 i-T-f 6606NZCL.995/JW/iv t! . SU' 1995 *1;-u, a;- 13-;248 2fl7;7;A process according to any one of claims 1 to 6, in which the hydrogen peroxide is at ambient temperature when it is added to the liquid.;5;8;A process according to any one of claims 1 to 7, in which means are provided for cooling the vessel.;10 9;A process according to any one of claims 1 to 8, in which the liquid is heated to between 30° and 50° C before addition of the hydrogen peroxide commences.;10;15 A process according to claim 1, wherein the amount of hydrogen peroxide which is added to the liquid equals between 103% and 110% of the stoichiometric amount of arsenic trioxide used in the production of the batch.;11;20 A process according to any one of claims 1 to 10, in which the arsenic trioxide is present in the body of liquid in a proportion of between 1 and 2 parts to 3 parts of water weight for weight before introduction of the hydrogen peroxide commences.;12;25 A process according to claim 3, wherein additional arsenic trioxide is added to the liquid after addition of the hydrogen peroxide has commenced and before addition of the hydrogen peroxide is complete.;13;30 A process according to claim 12, in which the additional arsenic trioxide is added in not more than 61 minutes after addition of the hydrogen peroxide has commenced.;14;A process according to claim 12 or claim 13, in which the total amount of arsenic 35 trioxide mixed with the water before addition of the hydrogen peroxide commences and;•rr---"-v.-.;added to the liquid after addition of the hydrogen peroxide commence^^m'al^eF'uR;/*' A' \ .,! of] 6606NZCL.995/JW/iv 2 J $£P j <] <] $ V>j 24 8 2R7 - 14- between 150% and 300% by weight of the original amount of water in the mixture. 15 5 A process for the production of aqueous arsenic acid, substantially as herein described and claimed. 16 Aqueous arsenic acid, whenever produced by a process according to any one of claims 10 1 to 15. 17 A process for preparing a copper chrome arsenate solution including the steps of adding Cr03 and CuO in suitable form to an aqueous solution of arsenic acid solution prepared 15 by a process according to any one of claims 1 to 15. A process for preparing a copper chrome arsenate solution including the steps of adding first Cr03 in suitable form and thereafter CuO in suitable form to an aqueous solution 20 of arsenic acid solution prepared by a process according to any one of claims 1 to 15. A process according to claim 17 or claim 18, in which the Cr03 and the CuO are in finely divided form. A process for preparing a copper chrome arsenate solution substantially as herein described and claimed. 18 19 25 20 30 J 20 Attorneys for the Applicant John Ayrton Branch. 35 6606NZCL.995/JW/iv