PROCESS FOR THE MANUFACTURE OF GRANULAR MONOHYDRATED SODIUM PERBORATE

Abstract: A process for the manufacture of granular monohydrated sodium perborate, having high abrasion resistance, starting from the corresponding tetrahydrated perborate, characterized in that a carboxylic acid, containing from 1 to 3 carbon atoms, or an alkali metal salt thereof, is added to the tetrahydrated perborate, already present in the granular state, whereafter dehydration of the tetrahydrated perborate is carried out.

Description: The invention relates to a process for the manufacture of granular monohydrated sodium perborate. Granular monohydrated sodium perborate (NaBO₂ · H₂O₂) is generally obtained by dehydration of the so-called tetrahydrated salt (NaBO₂ · H₂O₂ · 3H₂O) in the granular state and is mainly used as a bleaching agent for detergent formulations. The monohydrated salt essentially has three advantages in comparison with the tetrahydrated salt: a higher content of available oxygen, a higher heat stability and a higher dissolution rate into water. However, it is known that the monohydrated salt is much more brittle than the tetrahydrated salt. One of the main reasons which have hitherto limited its exploitation is that, as a consequence of the manufacture of the product, the brittleness involves a variation of the distribution of the particle size and an enriching of the powder with the finest fractions. It is undesirable to use a product having a high content of fine powder, in that it causes serious hygienic and ecological problems during the mixing of the components and can favour the caking of detergent when it is manufactured. In order to avoid the monohydrated salt, owing to its brittleness, being crushed by the pneumatic conveyors (everywhere employed by manufacturers and users) it is preferred to transport the monohydrated salt in bags rather than in bulk and all this involves higher burdens, as to either the packaging requirements or the reduced storage capacity. Hence the necessity of having a monohydrated salt having good abrasion resistance, which can be conveyed in bulk without producing any fine powder. In the past two main ways were undertaken: a) control of the dehydration conditions of the tetrahydrated perborate and in particular control of temperature and moisture of the drying air (see for instance European Patent Nos EP-155,894 and 194,952; U.S. Patent No. US-A-3,623,836; German Patent publication DE-A-2,444,780 and 2,258,319; and French Patent No. FR-1,081,421 or: b) addition of specific additives to the monohydrated perborate (see for instance German Patent No. DE-2,203,285) However, in both cases, it was impossible to obtain a product to be conveyed in bulk which would wholly satisfy the expectations of the users. Another way for overcoming the problem is indicated in European Patent No. EP-202,519; according to this patent, the use of additives (in this particular case sodium meta-silicate) is not performed on the finished monohydrated salt but on the tetrahydrated salt before its drying. In this way, the melting point of the tetrahydrated salt (otherwise 63°C) is lowered so that (during the starting step of dehydration on the particle surface) an initial melting takes place, by quick withdrawal of the molecules of crystallization water, which withdrawal gives the finished product the desired stiffness. In this way, it is possible to improve the abrasion resistance of the monohydrated perborate, by a simple way and independently of the drying method. However, the addition of meta-silicate does not allow a monohydrated perborate to be obtained having a particularly high content of available oxygen, because meta-silicate generally retains very firmly the crystallization water, which is not completely removed under the usual dehydration conditions of the tetrahydrated perborate. It has long been attempted to find new additives able to decrease the melting point of the tetrahydrated perborate (thus obtaining a high abrasion resistance of the corresponding monohydrate) and which does not contain any crystallization water at the end of the dehydration, thus ensuring a sufficiently high content of available oxygen in the final product. We have now succeeded in developing a process for obtaining granular monohydrated perborate having a good abrasion resistance, which can be also transported in bulk and which exhibits a high content of available oxygen. The present invention provides a process for the manufacture of granular monohydrated sodium perborate having high abrasion resistance, starting from the corresponding tetrahydrated perborate, characterized in that a carboxylic acid containing from 1 to 3 carbon atoms, or an alkali metal salt thereof, is added to said tetrahydrated perborate, already present as a granular product, and that a dehydration of the perborate is then carried out, according to conventional techniques, until substantially complete conversion to monohydrated perborate takes place. The carboxylic acid is preferably selected from formic acid, acetic acid and mixtures thereof. The alkali metal salt is preferably a sodium salt, preferably selected from sodium formate, sodium acetate (anhydrous or trihydrate), sodium propionate, and mixtures thereof. These salts are wholly compatible with monohydrated perborate and with other components, generally used in detergent formulations; furthermore, they are wholly soluble in water and do not affect in any way the solubility and dissolution rate of monohydrated perborate and of the detergent formulations. These salts can be used alone, in admixture among them or mixed with sodium silicates; in some cases the mixture represents an improvement, in comparison with the use of a single compound, as to the mechanical resistance of the monohydrated salt. This is especially true for acetate-formate mixtures. The total amount of carboxylic acid or salt, on the basis of the dry tetrahydrated perborate, is generally from 0.2 to 2% b.w.; amounts lower than 0.2% show a reduced effect on the abrasion resistance, whereas amounts higher that 2% do not cause any substantial improvement; on the contrary, they adversely affect the content of active oxygen of the final monohydrated salt. More preferably, the amount of dry carboxylate is from 0.25 to 1% by weight, on the dry tetrahydrated perborate. Salts of carboxylic acids can be added in the solid state or better as an aqueous solution. If the carboxylated salt is added in the solid state, it is preferable to add this salt finely ground (milled) to the wet granular tetrahydrated perborate; if the carboxylic salt or acid is added as an aqueous solution, this solution should be poured dropwise or sprayed on the dry or wet granular tetrahydrated perborate; however, in this last case, the total free water in the additioned tetrahydrated perborate should not be higher than 10% b.w., on the dry tetrahydrated perborate, because an exceedingly wet product would cause problems as to movement and fluidization. By working with an aqueous solution, concentrations of the additive (in the solution) generally range from 5 to 50% b.w.; however, the preparation of aqueous solutions of these compounds do not involve any problem, because they are very soluble in water. Too dilute solutions give rise to an exceedingly wet tetrahydrated perborate, which has the above mentioned drawbacks; too concentrated solutions, on the other hand, can give rise to a non-homogeneous distribution of the additive in the tetrahydrated perborate (and consequently lower the effect of the additive itself). More preferably, the operation is carried out using aqueous solutions having a carboxylic acid (or salt) content from 5 to 40% b.w., on the solution. In order to ensure a better distribution of the additive either dry or as a solution, it should be added to the tetrahydrate kept under agitation, in a suitable apparatus, such as a rotating drum, a blade mixer or a fluidized bed. The movement of the mass should be maintained for the time necessary to ensure a perfect mixing of the components. The dissolution of the carboxylic acid or salt into water and the addition thereof to sodium tetrahydrated perborate are generally carried out at room temperature, but the operation may be carried out at a higher or lower temperature, provided that, in the addition step, the temperature of 63°C, namely the melting temperature of sodium tetrahydrated perborate, is not exceeded. The drying of the tetrahydrated perborate to monohydrated salt can be carried out according to generally used techniques, under normal pressure or in vacuo, continuously or in batch, under static conditions or on a product maintained in movement. Generally, one can work in a fluidized bed dryer using hot-air, in one or more steps (in particular 1 or 4 steps connected in series). The temperature of the air entering the dryer is generally from 80 to 180°C; it is possible to use either air or moist air; in this last case, is is preferred that the moisture content be equal to or lower than 86 g of water per Kg of dry air (dew point equal to or lower than 50°C). The average temperature of the mass to be dehydrated is preferably from 45 to 63°C. More preferably, feed air entering the dehydration zone has a temperature from 90 to 160°C and a moisture content from 0 to 28 g/kg (dew point: 0-30°C), still more preferably from 15 to 28 g/Kg (dew point: 20-30°C). An advantage of the invention is the reduction of the fine product blown off by the air stream, this amount being bigger the higher the velocity of the air and the finer the starting tetrahydrated salt. During a fluidized bed drying, the blown off product generally accounts (at the most) for 5-15% of the loaded tetrahydrated salt and this product must then be separated by means of suitable devices (cyclones, sleeve filters, etc.) and added to the product or recycled. Now, it was noted that by use of a carboxylic acid or salt according to the process of the invention the amount of blown off product can be reduced by 50%. A further advantage of the granular monohydrated salt, obtained by the process according to the invention, is its compatibility with activators, softening agents and other generally used components of formulations based on perborate; see for instance European Patent No. EP-137,533 and Swiss Patent Application No. CH-04703/85-6. In the process according to the invention, at the outlet of the dehydration zone, preferably the moisture content is from 10 to 52 g/kg, the relative humidity is from 10 to 40% and the temperature is from 45 to 60°C, such conditions being represented by area ABCDE in figure 1 of the accompanying drawings. More preferably, at the outlet of the dehydration zone, the moisture content is from 20 to 40 g/kg, the relative humidity is from 20 to 36% and the temperature is from 47 to 57°C, such conditions being represented by area FGHKM in figure 2 of the accompanying drawings. The invention in another aspect provides granular and free flowing monohydrated sodium perborate, obtained according to the process of the invention and having the following properties: available oxygen titre: equal to or higher than 15.0% by weight; bulk density: equal to or higher than 0.55 g/cm³; crushing loss: equal to or lower than 25%; surface area: from 4 to 8 m²/g. The invention will be further described with reference to the following illustrative Examples. OPERATIVE CONDITIONS COMMON TO EXAMPLES 1-10 A sample of tetrahydrated sodium perborate from an industrial production (formula: NaBO₂ · H₂O₂ · 3H₂O; available oxygen titre 10.2%) was sprayed, at room temperature and in a rotating drum (stainless steel; capacity 1 liter), with an aqueous solution of a selected carboxylic campound; the mass, after the addition, was maintained under agitation for about further 15 minutes, until the homogenization was complete. The thus additioned salt was then dehydrated in a cylindrical fluidized bed drier made of glass (diameter: 85 mm, height: 500 mm, diameter of the distributor: 44 mm) with a flow of 5000 Nl/h of dry air, (relative humidity 5%, at 20°C) heated at a preestablished temperature. The sample was coming from a usual industrial production and showed the following characteristics: The monohydrated perborate obtained from each test was characterized by determining available oxygen content, bulk density, crushing loss, dissolution rate, surface area and distribution of the particle size. Available oxygen was determined by titration with 0.1 N potassium permanganate and bulk density was determined by measuring the mass of a sample freely poured into a cylinder of known capacity (without any settling). Crushing loss was determined on the fraction between 0.4 and 0.8 mm (sharp opening) following European Standard Practice Instruction No. 7/17050 of June 10, 1971, which simulate a pneumatic conveyance); the lower the value obtained by this crushing test, the higher the abrasion resistance of the sample. The dissolution rate was measured by the conductometric method on 1.5 g of product screened between 0.4 and 0.8 mm (sharp opening) in 750 cm³ of water, at 20°C, under agitation. Surface area was determined by the BET method and the distribution of the particle size was determined by means of a vibratory screen (50 g of the product for 10′ on screens of DIN series 4188). EXAMPLE 1 (Comparative blank test) A sample of 250 g of dry granular tetrahydrated Na perborate was dehydrated as such, without any previous addition of additive and carrying out the drying (in a fluidized bed) by means of 5000 Nl/h of dry air, pre-heated at 120°C for 70 minutes; the average temperature of the fluid mass, in the reactor, after about 20 minutes, was 54°C and the relative humidity in the reactor was 14%. The characteristics of the obtained product are recorded on Table 1. EXAMPLE 2 A solution obtained by dissolving 0.625 g of sodium formate (HCOONa) in 10 cm³ of water (concentration of the solution: about 6% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 0.25% b.w. of sodium formate on the starting tetrahydrated perborate, was dehydrated (as in example 1) by means of dry air (at 91°C) for 135′; the average temperature of the mass was 48°C and the relative humidity in the reactor was 12%. The characteristics of the monohydrated perborate are recorded on Table 1. As is clear, the crushing loss drastically dropped below the loss of Example 1. EXAMPLE 3 A solution obtained by dissolving 1.25 g of sodium formate in 10 cm³ of water (concentration of the solution: about 11% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 0.5% b.w. of formate on the starting tetrahydrate, was dehydrated as in Example 2, by means of 90°C air for 135′ (average temperature of the mass = 47°C, relative humidity = 12%). Properties of the thus obtained monohydrated perborate are recorded on Table I. EXAMPLE 4 A solution obtained by dissolving 1.25 g of anhydrous sodium acetate (CH₃COONa) into 10 cm³ of water (concentration of the solution: about 11% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 0.5% b.w. of sodium acetate on the starting tetrahydrate, was dehydrated as in Example 2 by means of 120°C air for 80′ (temperature of the mass = 49°C; relative humidity 18%). Properties of the thus obtained monohydrate are recorded on Table I. EXAMPLE 5 A solution obtained by dissolving 2.50 g of anhydrous sodium acetate (CH₃COONa) into 10 cm³ of water (concentration of the solution: about 20% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 1% b.w. of acetate on the starting tetrahydrate, was dehydrated as in Example 2 by means of 120°C air for 70′ (temperature of the mass = 49°C; relative humidity 18%). Properties of the thus obtained monohydrate are recorded on Table I. EXAMPLE 6 A solution obtained by dissolving 5.0 g of sodium acetate (CH₃COONa) into 10 cm³ of water (concentration of the solution: about 40% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 2% b.w. of sodium acetate on the starting tetrahydrate, was dehydrated as in Example 2 with 120°C air for 80′ (temperature of the mass = 49°C; relative humidity 15%). The characteristics of the thus obtained monohydrate are recorded on Table II. EXAMPLE 7 A solution obtained by dissolving 1.25 g of sodium propionate (CH₃CH₂COONa) into 10 cm³ of water (concentration of the solution about 33% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 0.5% b.w. of propionate on the starting tetrahydrate, was dehydrated as in example 2 with 120°C air for 70′ (temperature of the mass 52°C; relative humidity 16%). Properties of the thus obtained monohydrate are recorded on Table II. EXAMPLE 8 A sample of 260 g of moist sodium tetrahydrated perborate containing 4% by weight of free water, was intimately admixed with 4.15 g of trihydrated sodium acetate crystals (CH₃COONa.3H₂O). The thus additioned perborate, containing 1% b.w. of anhydrous acetate on the dry starting tetrahydrate, was dehydrated as in example 2 with 120°C air for 80′ (temperature of the mass 51°C; relative humidity 16%). Properties of the thus obtained monohydrate are recorded on Table II. EXAMPLE 9 A solution obtained by dissolving 1.25 g of anhydrous sodium acetate (CH₃COONa) and 1.25 g of sodium formate (HCOONa) into 10 cm³ of water (total concentration of the solution 20% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 0.5% b.w. of formate on the starting tetrahydrate, was dehydrated as in example 2 with 91°C air, for 135′ (temperature of the mass 47°C; relative humidity 12%). Properties of the thus obtained monohydrate, really excellent, are recorded on Table II. EXAMPLE 10 A solution obtained by dissolving 0.62 g of anhydrous sodium acetate (CH₃COONa) and 1.88 g of sodium formate (HCOONa) into 10 cm³ of water (total concentration of the solution 20% b.w.) was sprayed on a sample of 250 g of dry granular tetrahydrated Na perborate according to the previously described operative conditions. The thus additioned perborate, containing 0.75% by weight of formate and 0.25% by weight of acetate on the starting perborate tetrahydrate, was dehydrated as in example 2 with 91°C air for 135′ (temperature of the mass 46°C; relative humidity 13%). Properties of the thus obtained monohydrate, really excellent, are recorded on Table II. Operative conditions common to examples 11-16. Dry granular tetrahydrated Na perborate was sprayed, at room temperature and in said rotating drum, with an aqueous solution of a prefixed carboxylic acid. The subsequent operative conditions are as in the former examples, except that the amount and the humidity of the feed air are slightly (increased to a dew point from 0 to 30°C, namely a moisture content from 0 to 28 g per Kg of dry air) before pre-heating it to a prefixed temperature, which depends on each single test. EXAMPLE 11 300 g of dry and granular tetrahydrated Na perborate were sprayed with a solution obtained by dissolving 1.10 g of acetic acid into 10 cm³ of water (solution concentration = ca. 9.9% by weight). The thus additioned perborate, containing 0.36% by weight of acetic acid, was dehydrated for 40 minutes by means of 6000 N liter/h of air saturated with water at 30°C (moisture content: 28 g/Kg) and then heated at 140°C. The average temperature of the dehydrating mixture (at the outlet of the bed) was about 57°C and the corresponding relative humidity was 35% (moisture content: 38 g of H₂O per Kg of dry air); the properties of the obtained mono-hydrated perborate are recorded on Table III. EXAMPLE 12 300 g of the sample of example 11 were sprayed with a solution obtained by dissolving 2.16 g of acetic acid into 10 cm³ of water (solution concentration = ca. 17.7% by weight). The thus additioned perborate, containing 0.72% by weight of acetic acid, was dehydrated for 43 minutes by means of 6000 N liter/h of air saturated with water at 25°C (moisture content: 20 g/Kg) and then heated at 140°C. The average temperature of the dehydrating mixture was about 55°C and the relative humidity was 32% (moisture content: 32 g/Kg); properties of mono-hydrated perborate are recorded on Table III. EXAMPLE 13 300 g of the sample of example 11 were sprayed with a solution obtained by dissolving 1.00 g of formic acid into 10 cm³ of water (solution concentration = ca. 9.1% by weight). The thus additioned perborate, containing 0.33% by weight of formic acid, was dehydrated for 38 minutes by means of 6000 N liter/h of air saturated with water at 25°C (moisture content: 20 g/Kg) and then heated at 140°C. The average temperature of the dehydrating mixture was about 55°C and the relative humidity was 35%, (moisture content: 34 g/Kg); properties of mono-hydrated perborate are recorded on Table III. EXAMPLE 14 300 g of the sample of example 11 were sprayed with a solution obtained by dissolving 1.10 g of acetic acid and 1.00 g of formic acid into 10 cm³ of water (global solution concentration = ca. 17.3% by weight). The thus additioned perborate, containing 0.36% by weight of acetic acid and 0.33% by weight of formic acid, was dehydrated for 45 minutes by means of 6000 N liter/h of air saturated with water at 20°C (moisture content: 15 g/Kg) and then heated at 140°C. The average temperature of the dehydrating mixture was about 55°C and the relative humidity was 29%, (moisture content: 30 g/Kg); properties of mono-hydrated perborate are recorded on Table III. EXAMPLE 15 300 g of the sample of example 11 were sprayed with a solution obtained by dissolving 1.10 g of acetic acid and 1.00 g of formic acid into 10 cm³ of water (global solution concentration = ca. 17.3% by weight). The thus additioned perborate, containing 0.36% by weight of acetic acid and 0.33% by weight of formic acid, was dehydrated for 40 minutes by means of 6000 N liter/h of dry air (moisture content: 5 g/Kg) heated at 140°C. The average temperature of the dehydrating mixture was about 55°C and the relative humidity was 20% (moisture content: 22 g/Kg); properties of mono-hydrated perborate are recorded on Table III. EXAMPLE 16 300 g of the sample of example 11 were sprayed with a solution obtained by dissolving 1.20 g of propionic acid into 10 cm³ of water (solution concentration = ca. 10.7% by weight). The thus additioned perborate, containing 0.40% by weight of propionic acid, was dehydrated for 40 minutes by means of 6000 N liter/h of air saturated with water at 30°C (moisture content: 28 g/Kg) and then heated at 140°C. The average temperature of the dehydrating mixture was about 56°C and the relative humidity was 36% (moisture content equal to 38.4 g/Kg); properties of mono-hydrated perborate are recorded in Table III.

Claims: 1. A process for the manufacture of granular monohydrated sodium perborate having high resistance to abrasion, starting from the corresponding tetrahydrated perborate, characterized in that a carboxylic acid containing from 1 to 3 carbon atoms, or an alkali metal salt thereof, is added to said tetrahydrated perborate, already present in the granular state, and then carrying out dehydration of the tetrahydrated perborate until the conversion to monohydrated perborate is substantially complete. 2. A process as claimed in claim 1, characterized in that said carboxylic acid is selected from formic acid, acetic acid and mixtures thereof. 3. A process as claimed in claim 1 or 2, characterized in that said alkali metal salt is a sodium salt. 4. A process as claimed in claim 3, characterized in that said sodium salt is selected from sodium formate, anhydrous or trihydrated sodium acetate, sodium propionate and mixtures thereof. 5. A process as claimed in claim 3 or 4, characterized in that said sodium salt is a mixture of sodium formate and sodium acetate. 6. A process as claimed in any of claims 1 to 5, characterized in that the amount of carboxylic acid or of alkali-metal salt is from 0.2 to 2%, preferably from 0.25 to 1%, by weight, on the dry tetrahydrated sodium perborate. 7. A process as claimed in any of claims 1 to 6, characterized in that said carboxylic acid or said alkali metal salt are added in the dry state or as an aqueous solution to the tetrahydrated sodium perborate kept under agitation. 8. A process as claimed in claim 7, characterized in that said carboxylic acid or said alkali metal salt are added as an aqueous solution, the amount of acid or salt being from 5 to 50%, preferably from 5 to 40%, by weight, on said solution. 9. A process as claimed in any of claims 1 to 8, characterized in that feed air entering the dehydration zone has a temperature from 80 to 180°C and a moisture content equal to or lower than 86 g of water per kg of dry air (dew point equal to or lower then 50°C), and in that the average temperature of the mass to be dehydrated is from 45 to 63°C. 10. A process as claimed in any of claims 1 to 9, characterized in that feed air entering the dehydration zone has a temperature from 90 to 160°C and a moisture content from 0 to 28 g/kg (dew point: 0-30°C), preferably from 15 to 28 g/kg (dew point: 20-30°C). 11. A process as claimed in any of claims 1 to 10, characterized in that, at the outlet of the dehydration zone, the moisture content of the air is from 10 to 52 g/kg, the relative humidity is from 10 to 40% and the temperature is from 45 to 60°C, such conditions being represented by area ABCDE in figure 1 of the accompanying drawings. 12. A process as claimed in any of claims 1 to 11, characterized in that, at the outlet of the dehydration zone, the moisture content of the air is from 20 to 40 g/kg, the relative humidity is from 20 to 36% and the temperature is from 47 to 57°C, such conditions being represented by area FGHKM in figure 2 of the accompanying drawings. 13. Granular and free flowing monohydrated sodium perborate, obtained according to the process claimed in any one of the preceding claims and having the following properties: available oxygen titre: equal to or higher than 15.0% by weight; bulk density: equal to or higher than 0.55 g/cm³; crushing loss: equal to or lower than 25%; surface area: from 4 to 8 m²/g.

Inventors: CONDO, ANTONINO, FERRARIO, RENZO
Applicants: AUSIMONT S.P.A.
Application year: 1988
Main classification: C01B15
IPC codes: C01B15