The present invention relates to a process for the manufacture of peracids and more particularly to the manufacture of particulate peracids to permit their isolation and recovery from an acidic reaction medium.

Peracids, sometimes alternatively referred to as peroxyacids, as a class represent a range of potentially useful materials, since they are powerful oxidants, are effective wide spectrum biocides and are effective bleaches of ^" stains, both under industrial conditions and in the domestic environment. In a number of formulations, the peroxyacids are employed as described for employment in solid, particulate form. In consequence, it is desirable to devise processes for the manufacture of peracids that result in an isolatable particulate solid.

In one process for making peracids, an organic carboxylic acid substrate is brought into reactive contact with hydrogen peroxide in the presence of a strong acid such as a mineral acid like sulphuric acid or a corresponding organic acid such as methanesulphonic acid. Many of the carboxylic acid substrates are solid at convenient operating temperatures, that are often selected and controlled in the range of from 5 to 45°C and are poorly soluble in aqueous media. Likewise, many of the resultant peracids are also poorly soluble in aqueous media. Several variations have been descibed in current literature in the manner in which the carboxylic acid, the hydrogen peroxide and the strong acid are brought into contact with each other. In many of the variations, steps are taken to promote the reaction in the liquid phase, for example by dissolving the substrate in at least a fraction of the strong acid before it is brought into contact with the hydrogen peroxide. In other variations, the hydrogen peroxide is premixed with at least a fraction of the strong acid. Whatever the technique for mixing the reagents, it has

been recognised since at least USP 281 3896 (Krimm) that one important parameter is the ratio of strong acid to carboxylic acid, and that a high ratio of strong acid to carboxylic acid favours high conversion to the percarboxylic acid . This can be expressed as a high molar or weight ratio as in an article by Y L Zherebin et al in Zorka, vol 8, p41 -44 ( 1 972) that prefers a 20 fold molar excess of sulphuric acid over dicarboxylic acid. Alternatively, it can be expressed as the reaction mixture containing a high proportion of strong, eg sulphuric acid, often of over 60% w/w and in many instances up to about 80% w/w. The proportion of sulphuric acid compared with the total weight of sulphuric acid and water is sometimes alternatively called the A value.

Reaction mixtures having high A values tend to be able to dissolve both the carboxylic acid substrate faster and/or the resultant peracid product to a greater extent than can low A value reaction mixtures. As a consequence, a fraction of the peracid product, and in many instances a considerable fraction of the product remains in solution when the reaction is finished.

One method of improving the extent of recovery of solid peracid from a concentrated acidic solution is to lower the A value of the reaction mixture by introduction of water or ice, an operation that is often called quenching. In the course of conducting experimentation into the manufacture of peracids and especially diperacids that are only poorly soluble in aqueous media, it has been found that the method of quenching can affect to a significant extent the properties of the resultant solid product. These properties can include the physical properties of the solid that is produced, which in turn affect the rate at which the solids can be separated from the supernatant liquor in conventional separation methods, such as filtration and the proportion of liquor retained by the solids. It is inherently desirable to devise precipitation processes that produce a readily filterable product, because products that are poorly filterable can markedly increase product recovery times and/or require excessively large filtration equipment, both of which can increase significantly manufacture costs. Excessive liquor retention in the solids not only leads to potentially increased drying costs but also either increases the time taken for washing/purification and hence its costs or the level of residual and destabilising acidic impurity in the product. On the other hand, the chemical properties of the resultant peracid are also of considerable practical importance, such as the extent of reversion or partial reversion to either the carboxylic acid substrate or the half carboxylic acid half peracid in the case of a diperacid, since they

determine the extent of net conversion of the precursor to the peroxyacid and the effective base cost of the product per unit amount of peroxyacid available oxygen (avox).

As a general trend, from the dilution/recovery of the same peroxyacid, it has been found that the disclosed technique of dropping the reaction mixture containing sulphuric acid into a substantial volume of ice/water diluent such as double the reaction mixture volume dilutes to a desirable extent, but tends to produce an extremely fine particulate product that can blind filters and is slower to filter. The effect is observable not only on the initial filtration but especially during subsequent .washing to remove undesired acid impurities. On the other hand, it has been observed that a progressive slow dilution of the mixture with the same volume of diluent tends to alter the morphology of the product and to impair the chemical purity of the product, the latter effect possibly as a result of acid catalysed hydrolysis (reequilibration) of the product.

It is an object of the present invention to devise a process for the preparation of a peroxyacid in solid form from a solution thereof in a reaction mixture of high A value which avoids and/or ameliorates the twin problems of excessively slow rate of filtration and excessively impaired peracid purity.

In accordance with a first aspect of the present invention there is provided a process for the manufacture of particulate poorly water-soluble peroxyacid in which the peroxyacid is present in a aqueous solution of a strong acid having an A value of at least 0.6 which is brought into mixture with diluent aqueous material in an amount sufficient that the peroxyacid precipitates out of solution characterised in that contact with diluent aqueous material selected from water or dilute aqueous solution of strong acid and/or hydrogen peroxide is conducted in at least two stages of which the first comprises rapid dilution until a solid particulate precipitate is observable and thereafter slower dilution whilst further peracid precipitates out of solution resulting in growth of the precipitated particles.

In accordance with a related aspect of the present invention there is provided a process for the manufacture of particulate poorly water-soluble peroxyacid in which the peroxyacid which is dissolved in an aqueous solution of a strong acid having an A value selected in the range of from 0.6 to 0.85 is diluted with diluent aqueous material in an amount sufficient that the peroxyacid precipitates out of solution characterised in that contact with aqueous material selected from diluent water or dilute solution of strong acid

and/or hydrogen peroxide is conducted in at least two stages, the first stage comprising rapid dilution to reduce the A value by an amount selected in the range of from 0.07 to 0.25 A value units in order to attain rapidly an A valuecorresponding to the point of nucleation and the second stage comprising slower dilution during subsequent peracid precipitation to promote crystal growth.

By controlling the method of diluting the solution of peroxyacid in the reaction mixture, it is possible to improve the balance of properties of the resultant particulate peroxyacid solid, including its particle size, shape and size distribution and its residual peracid avox (available oxygen) content. Where the peroxyacid is a diperacid, the balance includes the distribution between diperacid, monoperacid and diacid species in the separable product.

The solution of peroxyacid before dilution will normally comprise a reaction mixture obtained by reacting a peroxyacid precursor, which usually would comprise the corresponding carboxylic acid, but which could alternatively comprise the corresponding anhydride or less commonly an ester, with concentrated aqueous hydrogen peroxide in a strong acid reaction medium. The reaction medium is advantageously a mineral acid such as especially sulphuric acid, by virtue of its ready availability and high acid strength or can if desired comprise an organic derivative such as methanesulphonic acid. The selection of the reaction conditions is at the discretion of the peroxyacid manufacturer, taking into account which peroxyacid is being made. Such conditions include specifically the acid strength of the reaction mixture, often referred to as the A value, which the weight proportion represented by the strong, eg sulphuric acid, of the total weight of sulphuric acid, plus water in the reaction mixture. The A value in the reaction mixture is normally at least 0.6 and is often not higher than 0.9. The inverse relationship of a preferred A value to the inherent solubility of the precursor carboxylic acid is disclosed for example in International Publication No WO 90/14336. In general, it is preferred to employ an A value of between 0.7 and 0.8 to balance the twin constraints of operational safety and reactivity.

There are several variations in the methods of bringing together peroxyacid precursor and the hydrogen peroxide reactants in order to produce a reaction mixture containing eventually peroxyacid, all of which can be employed at the discretion of the manufacturer for the formation of a peroxyacid solution from which particulate peroxyacid product by a process according to the present invention, and many of which have been described

in published literature. Such variations include the dissolution of the peroxyacid precursor in all or a fraction of the strong acid (eg sulphuric acid) and the slow addition thereto of concentrated hydrogen peroxide. In other variations, a fraction of the sulphuric acid is premixed with the aqueous 5 hydrogen peroxide solution which is then mixed with the solution of precursor in sulphuric acid, for example in a controlled manner such as described in aforesaid WO 90/14336 (eg Example 8) or possibly in a continuous process. In other variations, the peroxyacid precursor is employed in the form of a particulate solid which is progressively introduced

10 into the premixture of sulphuric acid and hydrogen peroxide.

In a further variation employing a particulate carboxylic acid precursor which variation is the subject of a copending application of even date, the reaction medium is produced in two stages, in the first of which a reaction mixture of lower A value is produced which is increased to a higher A value

15 in the presence of or immediately prior to contact with the particulate carboxylic acid.

In the foregoing reactions, the reaction temperature is usually controlled to within the range of from about 5°C to about 50°C, and in many instances from about 1 5 to about 35°C. The hydrogen peroxide is usually employed

20 in excess over the stoichiometric amount, ie at greater than 1 mole per equivalent mole of carboxylic acid (or corresponding anhydride or ester) prescursor and often from about 2 to 5 moles per equivalent mole. Its concentration in the liquid phase of the reaction medium is often initially from about 3 to 20% by weight, depending in part on the ratio of the total

25 weight of hydrogen peroxide/sulphuric acid/water to the peroxyacid precursor. Such ratio is normally at least about 4: 1 and usually not higher than about 30: 1 . Although the selection of that ratio is at the discretion of the peroxyacid manufacturer, for the manufacture of the particularly preferred aliphatic diperoxyacids containing an embedded phenylene group

30 described herein, a preferred range is from about 5.5: 1 to about 9: 1 and especially when employing carboxylic acid precursor in particulate form. In accordance with recognised procedures, the reaction mixture can contain, if desired, a small fraction such as 0.1 to 5 gpi of known stabiliser(s) for hydrogen peroxide and/or peroxyacids such as picolinic acid,

3.1 dipicolinc acid, and/or organopolyphosphonates.

As a result of the peroxidation process, there is produced a reaction mixture containing peroxyacid in solution. The invention process is

applicable particularly to reaction mixtures which are free from any solids, be they peroxyacid or peroxyacid precursor.

Preferably, the peroxyacid that is produced in particulate form by a process according to the present invention is the peroxidised product or products obtained by reacting hydrogen peroxide with a dicarboxylic acid substrate of formula H0 ₂ C-R ^, -(NR) _a -CO-(NR) _b -R"-(NR) _c -CO-(NR) _d -R ^{, ,,} -C0 ₂ H of which a particularly desired product comprises the diperoxyacid of formula HO ₃ C-R ^, -(NR) _a -CO-(NR) _b -R"-(NR) _c -CO-(NR) _d -R ^, "-CO ₃ H in which formulae R represents hydrogen, alkyl, aryl, or alkaryl or aralkyl to 12 carbon atoms, a -f- b = 1 , c -ι- d = 1 , R' and R'" each represents a methyiene or polymethylene group of from 1 to 10 carbons that is optionally substituted by an aryl group and R" represents an arylene or a polymethylene group containing up to 10 carbons, or together with an adjacent nitrogen an aromatic heterocyclic group. The number of carbons in the compound of formula 1 is often selected in the range of from 1 2 to 30 and in some especially preferred compounds is from 18 to 22, and particularly 20. It is especially preferred for R to represent hydrogen and particularly preferable for a and d to each equal 1 , ie b and c to equal 0 and especially if R" is arylene. In compounds according to the formula, the aryl group R, R' or R'"or arylene group R" can, if desired, be substituted by a halo, alkyl group or further aryl group, ie examples of groups which are not themselves oxidised or removed during the reaction process in which the peroxyacid is produced and in a number of compounds are preferably respectively phenyl and phenylene. R' and R' " are each preferably selected from the group of from trimethylene up to hexamethylene, and an especially convenient example is pentamethylene. It is especially desirable for R" to be meta or para phenylene.

In a number of especially convenient embodiments, R is hydrogen, a and d are each 1 , R' and R'" are each pentamethylene, and R" is meta or para phenylene.

Other peroxyacids which can be contemplated for production in solid form according to the present invention comprise the polar amide-containing monoperoxyacids of USP 4686063 (Burns) to the extent that such monoperoxyacids are sufficiently stable for them to be produced and recovered and the amido monoperoxyacids of USP 5098598 (Sankey et al) . The process in which the peroxyacid-containing reaction mixture is diluted in at least two stages is often conducted at a temperature of from about 0°C to about 30°C. The temperature of the liquid diluent is usually

sub-ambient, and often chosen in the range of from approximately its freezing point , which is typically about 0°C, up to 10°C and in many instances is around 5°C.

The first stage dilution can be conducted in two ways. In one way, the diluent is introduced progressively but quickly into the reaction mixture. The diluent in the first stage can comprise water or a comparatively dilute solution of the strong acid, for example a solution of at least 0.3 A value below the A value of the reaction mixture, in which a fraction of the water/solution can comprise ice, preferably crushed. The amount of diluent to employ can be determined by introducing the diluent until a solid precipitate become apparent or by using the proportionate amount determined from a prior ranging experiment. In a closely related variation of this way, the aforesaid proportionate amount can be added in a single shot, desirably with agitation to improve the mixing of the diluted mixture. In a second method, and following determination of a suitable volume of diluent to employ in the first stage as indicated above, the reaction mixture can be introduced into the proportionate amount of diluent. It will be understood that A value at which precipitation is observed represents an optimum and that the benefit of the multi stage dilution process is attained to a considerable extent when the amount of diluent introduced in the first stage produces an A value that is close to the optimum, for example within + /- 0.025 A units.

In practice it will be recognised that the rate of dilution in the first stage is constrained by the rate at which one fluid can be transferred safely and mixed with a second fluid . The rate of dilution, as measured by the change in A value of the mixture is generally at least 0.025 A value units per minute and in many instances is from 0.05 to 0.2 A value units per minute. The dilution period in the first stage often lasts from about 5 seconds to about 2 minutes, but the minimum duration can increase when large volumes of diluent are introduced in accordance with chemical engineering practice.

In a number of operating embodiments, and especially when employing preferred diperoxyacids of formula 1 , the A value of the first stage diluted mixture is selected in the range of from 0.55 to 0.65.

The subsequent dilution can be carried out in at least one further stage, and in some particularly desirable embodiments in two stages, the rate of dilution being slower in the second stage than in the first stage. The second stage slow dilution applies particularly to the period from initial precipitation of some peracid to the point at which most of and especially

substantially all the peracid that is capable of being precipitated has, in fact, precipitated. Desirably, the slow rate of dilution of the second stage is continued until at least 90% and especially until at least 95 % of the precipitatable peracid has precipitated. One convenient way of defining the starting and finishing points for the second and slow dilution stage is with reference to the A value of the mixture and hence the slow stage lasts for the period taken to dilute from the one a value to the second . Such difference in A values is often rather small compared with the total overall extent of dilution, but the value of controlling the dilution during that period is high. The second stage A values difference is often selected in the range of from about 0.03 to about 0.06 A value units. It will be understood that it is possible for the second stage dilution rate can, if desired, be continued until the composition has reached its intended extent of dilution or alternatively a different dilution rate can be employed after the composition has attained an A value at which at least 90%, preferably at least 90% of the precipitatable peracid has precipitated.

During the second stage of dilution, the rate of dilution is slower than in the first stage and often, though not exclusively at least 5 times slower. In a number of desirable embodiments the rate of lowering of the A value is not greater than 0.02 A value units per minute and in many instances the rate is selected in the range of from about 0.005 to about 0.01 5 A value units/minute.. In practice, the diluent in the second dilution stage or stages is often water, dilute hydrogen peroxide or a dilute solution having an A value of up to about 0.2. In a preferred method of operation of the dilution process, the slow second stage rate of dilution lasts solely during the period of the main precipitation of the peracid and subsequent dilution is made at any convenient rate. This rate can, if desired, be similar to the rate of the first dilution stage, or can be an intermediate rate between the two preceding stages. This variation enables the advantage of the two stage process to be retained whilst accelerating the later part of the dilution, thereby improving plant utilisation and improving operating costs. Without being bound by any theory, it is believed that the rate of change of peroxyacid solubility with change in A value is very noticeable in the narrow region below the A value at which precipitation is noticeable on dilution, so that most of the peracid is precipitated within a narrow change in A value, but tends to be much smaller at changes in lower A values of for example below about 0.5 A value. On the basis of the experimentation in the programme

leading to the instant invention, it was deduced that the tendency for impurity formation is especially sensitive for peracid that is present in solution and less sensitive for peracid that has precipitated out of solution. The rate of dilution in the second stage for the passage from the A value for onset of peracid precipitation to its substantial completion is often controlled so that the period of the second stage is selected in the range of from 5 to 20 minutes.

Conveniently, the rate of dilution in the third stage can be at least half up to the rate of dilution in the first stage. The rate of dilution in the third stage, where it is higher than in the second stage, can be attained by a gradual increase of the dilution rate or may be attained in a step change. In many instances, the duration of the third stage is selected in the range of from 1 to 20 minutes.

The overall dilution process is, in many embodiments selected within the range of from about 20 to about 40 minutes.

The A value after all dilution stages is often selected in the range of about 0.3 to about 0.5, the exact figure depending amongst other factors upon the solubility profile of the peroxyacid relative to the A value of the mixture under the prevailing temperature and other operating conditions. It will be recognised that an alternative way of regarding the instant invention comprises the concept of a fast overall dilution in which the rate of dilution is slowed significantly during the period of precipitation of the peracid.

The dilution process can be subjected to external cooling, if desired. This can be effected by traditional means, such as for example a cooling jacket around the vessel in which dilution is occuring or the immersion of cooling coils within the mixture.

In a further variation, the dilution process can be conducted, if desired, using at least one diluent containing added hydrogen peroxide, either in the presence or absence of sulphuric acid. The peroxide can be added in either or all dilution stages. By so doing, it is possible to prevent to some extent re-equilibration of the peroxyacid species that remain in solution and consequently improve useful peroxyacid recovery. The concentration of hydrogen peroxide in the diluent or diluents is preferably no higher than the concentration that would be employed in the reaction mixture on recycle of the recovered diluted reaction medium. In a number of embodiments the peroxide is added at a concentration of from 0.5 to 5% w/w. It is advantageous to recycle such diluted reaction medium and especially if

hydrogen peroxide has been added in the dilution stage(s) since by so doing there is a reduced need for wasteful and costly disposal of reactants/reaction medium. In the presence of added hydrogen peroxide it is preferable to avoid high operating temperatures if the reaction medium is concentrated, ie by water removal, before it is recycled.

Dilution of a highly acid reaction mixture with water or an aqueous diluent to precipitate particulate peracid results in some impairment of the peracid purity and the manner of dilution affects the particle characteristics of the product. However, by employing the multi stage process of the present invention it is possible to control the extent of impairment of the of the purity as measured by avox content and proportion of di to monoperoxyacid (for diperoxyacid products), whilst producing the product in a particulate form that can be filtered reasonably quickly. Further, the multi stage route assists in the production of a peroxyacid product with a narrower particle size distribution, which can be of benefit for controlling the rate dispersion of the product.

Having described the invention in general terms, specific embodiments thereof will be described in greater detail by way of example only. Comparisons A and B In these comparisons not according to the present invention., a reaction mixture containing H0 ₃ C-(CH ₂ ) ₅ -NH-CO-C ₆ H ₄ -CO-NH-(CH ₂ ) ₅ -C ₃ OH (TOPCAP) as the predominant peracid was obtained by introducing a solution of the corresponding dicarboxylic acid (20g) in 98% sulphuric acid (40g) into a solution of hydrogen peroxide (85% w/w, 20.4g) in sulphuric acid (36g) and ice (23.6g) over a period of 30 minutes, the reaction mixture being kept to ambient (about 23-25°C) by a water jacket/bath to produce a reaction mixture having an A value of 0.725.

The resultant reaction mixture containing the peroxyacid was then split into two halves, each of which was subjected to a different dilution and washing process. In Comparison A, the product solution ( 1 10g) in a stirred flask was quenched by the addition within a few seconds of ice, 75g, to produce a dilute mixture having an A value of 0.294. A solid precipitate was observed which was filtered off using a filter funnel of approximate diameter 9cm fitted with Whatman 541 filter paper under suction from a standard water pump. Filtration took 4.5 minutes. A sample of 10 grams of the solids was washed with 800 mis of laboratory demineralised water in the same equipemnt. The washing took 23.8 minutes and achieved pH 1 .9. The solids were then washed again using a further 800 mis of water,

which took a further 45 minutes and achieved pH 4. The resultant double washed product was dried and on analysis had a purity of 89% (its measured avox content) was 6.68%, being 89% of the theoretical avox content of 7.55% for 100% pure TOPCAP. In Comparison B, the same weight of product solution (weight) was quenched by the slow introduction into the product solution in a stirred flask of the same weight of iced water as employed in comparison A, employing a perstaltic pump and an introduction period of 60 minutes. After 20 minutes addition of diluent, precipitation of solids was observed. The solids were recovered using the same equipment as in Comparison A, and thereafter a 10g sample was twice washed with 800 mis water in the same equipment as in comparison A. The initial filtration took 2.5 minutes and the subsequent washings took respectively 14.2 minutes and 32 minutes. All of these times are significantly quicker than in Comparison A, demonstrating that the dilution route of comparison B produced a product that was more readily filtered and washed. Analysis of the dried product indicated that the purity was only 81 %, (Avox of 6.1 1 %), an excessive and undesirable impairment in purity, and further indicated that the resultant separated product in B compared with A contained a smaller proportion of diperacid and conversly a higher proportion of unperoxidised or partially peroxidised material.

Comparisons C and D and Example 1 In these comparisons and Example, a batch of TOPCAP in a sulphuric acid reaction medium was produced by introducing a solution of the corresponding dicarboxylic acid (30g) in 98% sulphuric acid (90g) into a solution of hydrogen peroxide (85% w/w, 27.55g) in sulphuric acid (25.85g) and ice/water (36.62g) over a period of 30 minutes, the reaction mixture being kept to ambient (about 23-25°C) by a water jacket/bath to produce a reaction mixture having an A value of 0.725. The batch was divided into three equal fractions that were subjected to a different method of producing particulate peroxyacid product.

In the method of Comparison C, the reaction mixture was dropped into and stirred rapidly with 1 1 7.85g iced/water. The A value of the mixture fell straightaway to 0.3125 and solid particles precipitated immediately. The particles were small and resembled fine ribbons. The solid product was filtered in the same manner and apparatus as in Comparisons A/B and the filter cake was washed on the filter by passage of demineralised water until the effluent water had a pH of 3.0, which is indicative of a substantial

removal of retained sulphuric acid from the filter cake. The rate of flow of wash water was adjusted to maintain a substantially constant head of water above the filter cake. The volume of water needed to attain pH 3 was 1020 mis and the time taken was 140 minutes. The washed product was dried and was found to have an avox purity of 85 %.

In Comparison D, the reaction mixture was diluted by progressive, but rapid addition of the same volume of ice/water over a period of a few seconds. The diluted mixture reached the same A value as in Comparison C. Particulate TOPCAP precipitated out of solution and was filtered off and washed in the same way and using the same apparatus as for Comparison

C. Washing until the filtrate reached pH 3.0 took 960 mis and required 140 minutes of washing. The purity of the washed and dried product was 85%.

In Example 1 , the reaction mixture was diluted by a multi stage method, employing in total the same volume of ice/water as employed in each of Comparisons C and D. In the first stage, the reaction mixture was diluted by the rapid addition of the ice/water with stirring during less than 3 seconds until the mixture attained an A value of 0.618. Precipitation was observed. In the second stage, a second portion of the ice/water diluent (5ml) was introduced gradually over 5 minutes. The remainder of the diluent ice/water was then introduced over 7 minutes to attain a final A value of about 0.31 , substantially the same A vaue as in Comparisons C and

D. It was subjected to the same filtering and washing procedure using the same apparatus. Washing required only 540 mis to attain a filtrate effluent pH of 3.0 and a washing time of only 30 minutes. The washed and dried product had a purity of 82%.

From a comparison of Example 1 with the Comparisons, it will be observed that the product was very much easier and quicker to wash to an acceptable sulphuric acid residual and also required much less wash water, demonstrating that the control of the dilution during stage 2 was crucial.