PROCESS FOR THE PRODUCTION OF GRANULAR SODIUM PERBORATE MONOHYDRATE HAVING GOOD MECHANICAL STRENGTH

Abstract: The invention relates to a process for the produc­ tion of granular sodium perborate monohydrate having good mechanical strength. The process is characterized in that sodium silicate having an SiO₂/ Na₂O molar ratio com­ prised within the range of from 0.4 to 2.0 is added to granular sodium perborate tetrahydrate while this is being kept in movement and the so processed perborate tetrahydrate is dried until perborate monohydrate is obtained.

Description: Background of the Invention 1. Field of the Invention The present invention relates to a process for the production of granular sodium perborate monohydrate having good mechanical strength. 2. Description of the Prior Art Granular sodium perborate monohydrate (NaBO₂.H₂O) is obtained by dehydrating the corresponding granular tetrahydrate (NaBO₂.H₂O.3H₂O). Relatively to the tetrahydrate, perborate monohy­ drate shows essentially three advantages: a higher titer in active oxygen, a higher heat stability and a greater solution rate. It is however known that perborate monohydrate is considerably brittler than the tetrahydrate, and this is one of the main reasons which so far have limited its use. Such a drawback is in fact particularly detrimental, because it leads, during the handling of the product, to a change of its granulometric distribution, with an in­ crease in finer fractions. The fine product creates dust during its use, and hence causes problems of environmen­ tal hygiene. Furthermore, while perborate tetrahydrate is usually moved in bulk, this cannot be realized with perborate monohydrate which would be easily crushed by the pneumat­ ic conveying plants, utilized, at present, everywhere. For the reason above stated, monohydrate is moved only in bags, with higher packaging costs and reduced storing capacity. The Present Invention An object of the present invention is to provide a process for the production of granular sodium perborate monohydrate having good mechanical strength and, there­ fore, movable in bulk. This and still further objectsare achieved by the process of the present invention for the production of granular sodium perborate monohydrate having good me­ chanical strength. This process is characterized in that sodium silicate having an SiO₂/Na₂O molar ratio comprised within the range of from 0.4 to 2.0 is to granular sodium perborate tetrahydrate while this is being kept in move­ ment. The above mentioned addition can be carried out either by wetting granular sodium perborate tetrahydrate, while this is being kept in movement, with an aqueous solution of sodium silicate or by thoroughly mixing so­ dium silicate, in solid state, with granular sodium per­ borate tetrahydrate. The so treated tetrahydrate is then conveniently dried so much that perborate monohydrate is obtained. The wetting phase, with aqueous solutions of sodium silicate, can be carried out through known methods such as spraying, dropping, nebulization, etc. The amount of added sodium silicate, computated as the anhydrous salt (i.e., as the sum of SiO₂ and Na₂O) relatively to sodium perborate tetrahydrate is generally comprised within the range of from 0.25 to 2.5% by weight. Preferably, an amount thereof of from 0.5 to 1.5% is used. In the case that sodium silicate is added in aqueous solution form, the latter can have a concentration com­ prised, for example, between 5 and 50% by weight. In the case that the mixing of the two salts is carried out in the solid state, it is preferable to add dry sodium silicate, in very fine granular state, to damp granular sodium perborate, so as a more perfect diffusion of the additive compound on the perborate crystals sur­ face can be realized. As damp perborate, a product con­ taining generally up to 10% by weight of water is refer­ red to. The wetting or mixing of perborate tetrahydrate with sodium silicate are usually carried out at room tempera­ ture. However, also operating at a temperature higher or lower than room temperature is possible, provided such operating temperature be lower than 63°C, which is the melting point of sodium perborate tetrahydrate. The wetting or mixing are carried out on perborate tetrahydrate which is being kept in movement, for ex­ ample, by a rotary drum. It is preferable that after the end of the addition the mass be further kept in movement, either in the same equipment wherein the spraying took place, or in another device, in order to secure a homoge­ neous distribution of the product added to the granules of perborate. The drying of sodium perborate tetrahydrate pro­ cessed according to the present invention is carried out according to the known modalities for the drying of perborate tetrahydrate to the monohydrate salt. It can be carried out under stationary conditions (with high-tempe­ rature air and/or under vacuum) or, in one or more steps, on the product kept moved (e.g., on a fluid-bed dryer, on a vibrating-bed dryer, or in a rotary drum). The opera­ tion can be done, e.g., in a rotary vacuum dryer at temperatures comprised within the range of from 25° to 90°C. As it has already been mentioned, the SiO₂/Na₂O molar ratio of the sodium silicate is comprised within the range of from 0.4 to 2.0. Preferably, said ratio is comprised between 0.5 and 1.5. The molar ratio 1.0, which corresponds to a solution of sodium metasilicate, is particularly suitable. In this case use can be made both of anhydrous sodium metasilicate and of hydrated sodium metasilicate in any hydration form. The granules of perborate tetrahydrate can also be treated with an aqueous solution of H₂O₂ before being added with the sodium silicate. This pretreatment by H₂O₂ allows the slight decrease in active oxygen titer due to the addition of sodium silicate to perborate to be com­ pensated for. The amount of added H₂O₂, calculated as anhydrous hydrogen peroxide, is usually comprised within the range of from 0.1 to 1% by weight, relatively to perborate tetrahydrate. The concentration of the H₂O₂ aqueous solution is comprised e.g. within the range of from 5 to 60% by weight. It is usually sprayed at room temperature; also operating at a temperature lower or higher than room temperature is possible, provided the operating temperature be lower than 63°C. The use of sodium silicate, according to the present invention, yields further advantages besides that of decreasing the brittleness of sodium perborate monohy­ drate . First of all, it is perfectly water-soluble, so that the solubility of the perborate monohydrate gran­ ules is not decreased. On the other hand, sodium silicate is perfectly compatible with perborate monohydrate and with the other components used in the detergent formula­ tions. A further advantage of sodium silicate is that it increases the stability of perborate in solution, thus increasing the time of active oxygen release; the time during which the perborate exerts its action during the washing process results hence increased. The following examples are supplied to the purpose of better illustrating the inventive concept of the pres­ ent invention. Example 1 A sample of 200 g of sodium perborate tetrahydrate having a titer in active oxygen of 10.00% is sprayed, at room temperature, in a stainless-steel rotary drum of 1 litre of capacity, with 10 g of an aqueous solution at 20% by weight of sodium silicate having an SiO₂/Na₂O molar ratio equal to 1. The amount of solution is such as to correspond to the addition of 1.0% of sodium silicate as referred to perborate tetrahydrate. The mass is then kept in movement for one further hour to guarantee a complete homogenizing. The additive-containing product is subsequently dehydrated in a rotary dryer by operating under a resi­ dual pressure of 15 mm Hg : the temperature is gradually increased from 25° to 90°C within a 30 minutes time and is kept constant at 90°C for a further 30 minutes. The so obtained perborate monohydrate is charac­ terized by determining its titer in active oxygen, its brittleness, its bulk density, its solution rate and its stability in solution. The brittleness is determined on the fraction of 0.4-0.8 mm of mesh opening by the test as of European Laboratory Standard Practice Instructions No. 7/17050 of June 10th, 1971. The higher the brittleness value, the more fragile the product. The solution rate is measured by a conductometric method on 1.5 g of product screened between 0.4 and 0.8 mm of mesh opening, placed in 750 ml of water at 20°C under stirring: when the conductivity of the solution remains constant, all the product has been dissolved. The stability in solution is determined at 90°C on a 1% solution of perborate monohydrate submitted to a two hours stirring: the residual active oxygen at the end of the test is measured. The results of the characterization are the follow­ ing: Example 2 An identical sample of perborate tetrahydrate is processed as in Example 1, with the exception that, to comparison purpose, no addition has been carried out. The data obtained from the analysis of the so ob­ tained additive-free product are the following: Example 3 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. The spraying is carried out with 9 g of aqueous solution at 11% by weight of sodium silicate having an SiO₂/Na₂O ratio of 1. This corresponds to the addition of 0.5% by weight of sodium silicate as referred to perbo­ rate tetrahydrate. The characteristics measured on the obtained perbo­ rate monohydrate are the following: Example 4 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. The spraying is carried out with 10 g of aqueous solution at 25% by weight of sodium silicate having an SiO₂/Na₂O molar ratio of 0.67. This corresponds to the addition of 1.3% by weight of sodium silicate as referred to perborate tetrahydrate. The analytical data measured on the perborate mono­ hydrate obtained are the following: Example 5 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. Two sequential sprayings are carried out.The first spraying is carried out with 5 g of aqueous solution at 8% by weight of H₂O₂, corresponding to the addition of 0.2% by weight of H₂O₂ as referred to perborate tetrahy­ drate. The second spraying is carried out with 5 g of solution at 32% by weight of sodium silicate having an SiO₂/Na₂O molar ratio of 0.62, corresponding to the addi­ tion of 0.8% by weight of sodium silicate as referred to perborate tetrahydrate. The analytical data measured on the obtained perbo­ rate monohydrate are the following: Example 6 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. The spraying is carried out with 10 g of an aqueous solution at 20% by weight of sodium silicate having an Si₂O/Na₂O molar ratio of 2, corresponding to the addition of 1.0% by weight of sodium silicate as referred to perborate tetrahydrate. The analytical data measured on the obtained perbo­ rate monohydrate are the following: Example 7 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. The spraying is carried out with 9.5 g of an a­ queous solution at 21.1% by weight of sodium silicate having an Si₂O/Na₂O molar ratio of 0.49, corresponding to an addition of 1.0% by weight of sodium silicate as referred to perborate tetrahydrate. The characteristics measured on the obtained perbo­ rate monohydrate are the following: Example 8 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. The spraying is carried out with 4.4 g of an a­ queous solution at 45.81% by weight of sodium silicate having an Si₂O/Na₂O molar ratio of 1.64, corresponding to an addition of 1.0% by weight of sodium silicate as referred to perborate tetrahydrate. The characteristics measured on the perborate mono­ hydrate obtained are the following: Example 9 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. The spraying is carried out with 4.15 g of a solu­ tion at 47.45% by weight of sodium silicate having an Si₂O/Na₂O molar ratio of 1.35, corresponding to an addi­ tion of 1.0% by weight of sodium silicate as referred to perborate tetrahydrate. The characteristics measured on the obtained perbo­ rate monohydrate are the following: Example 10 The test is carried out as in Example 1, with the exception that the specifically described modalities have been used. The spraying is carried out with 4.0 g of a solu­ tion at 48.70% by weight of sodium silicate having an Si₂O/Na₂O molar ratio of 1.10, corresponding to an addi­ tion of 1.0% by weight of sodium silicate as referred to perborate tetrahydrate. The characteristics measured on the obtained perbo­ rate monohydrate are the following: Example 11 A sample of 208 g of damp sodium perborate tetrahy­ drate having a content of water of 3.8% is mixed, at room temperature, with 2 g of anhydrous sodium silicate in powdery state having an SiO₂/NaO₂ molar ratio equal to 1, in a stainless-steel rotary drum of 1 litre of capacity. The addition is such as to correspond to 1.0% as referred to dry perborate tetrahydrate.The mass is then kept in movement for one further hour to guarantee a complete homogenizing. The drying step to monohydrate is carried out ac­ cording to the method described in Example 1. The characteristics measured on the obtained perbo­ rate monohydrate are the following: Example 12 The test is carried out as in Example 11, with the exception that the specifically described modalities have been used. The mixing is carried out with 2 g of anhydrous sodium silicate in powdery state having an Si₂O/Na₂O molar ratio equal to 1.3, corresponding to the addition of 1.0% by weight of sodium silicate as referred to dry perborate tetrahydrate. The characteristics measured on the obtained perbo­ rate monohydrate are the following:

Claims: 1. Process for the production of granular sodium perbo­ rate monohydrate having good mechanical strength characterized in that sodium silicate having an SiO₂/Na₂O molar ratio comprised within the range of from 0.4 to 2.0 is added to granular sodium perborate tetrahydrate which is being kept in movement, and the so processed perborate tetrahydrate is dried until perborate monohydrate is obtained. 2. Process according to claim 1 characterized in that sodium silicate is added to granular sodium perborate tetrahydrate in aqueous solution form. 3. Process according to claim 1 characterized in that sodium silicate is added to granular sodium perborate tetrahydrate in solid state, preferably in very fine granules state. 4. Process according to claim 2 characterized in that the sodium silicate aqueous solution has a concentra­ tion comprised between 5 and 50% by weight. 5. Process according to claim 3 characterized in that sodium silicate is added in solid state to damp granular sodium perborate tetrahydrate. 6. Process according to preceding claim 5 characterized in that the amount of sodium silicate, as referred to sodium perborate tetrahydrate is comprised within the range of from 0.25 to 2.5% by weight. 7. Process according to claim 6 characterized in that the amount of sodium silicate, as referred to sodium perborate tetrahydrate is comprised within the range of from 0.5 to 1.5% by weight. 8. Process according to one or more of preceding claims characterized in that sodium silicate has an SiO₂/Na₂O molar ratio ranging from 0.5 to 1.5%. 9. Process according to claim 8 characterized in that the solution of sodium silicate has an SiO₂/Na₂O molar ratio of 1. 10. Process according to one or more of preceding claims characterized in that the granular sodium perborate tetrahydrate is treated with an aqueous solution of H₂O₂ before being added with sodium silicate. 11. Process according to claim 10, characterized in that the amount of H₂O₂ is comprised within the range of from 0.1 to 1% by weight, relatively to sodium perbo­ rate tetrahydrate. 12. Sodium perborate monohydrate obtained according to the process as described in one or more of preceding claims.

Inventors: CONDO' ANTONINO, FERRARIO, RENZO
Applicants: AUSIMONT S.P.A.
Application year: 1986
Main classification: C01B15
IPC codes: C01B15