GB2359546A

GB2359546A - Agglomerating sodium perborate monohydrate particles

Info

Publication number: GB2359546A
Application number: GB0004406A
Authority: GB
Inventors: Igan Hayati
Original assignee: US Borax Inc
Current assignee: US Borax Inc
Priority date: 2000-02-24
Filing date: 2000-02-24
Publication date: 2001-08-29
Also published as: GB0004406D0; EP1257499A1; AU2001237545A1; WO2001062664A1

Abstract

A method for the manufacture of sodium perborate monohydrate having a high bulk density (e.g. at least 0.7 g/cc) comprises agglomerating fine sodium perborate particles using an agglomerating agent a high concentration hydrogen peroxide solution (at least 20% w/v) and subsequently drying. The product obtained may have an active oxygen content of 16-18 % by wt. and is suitable for use in so-called compact or super compact detergent formulations.

Description

SODIUM PERBORATE MONOHYDRATE PARTICLES AND THEIR MANUFACTURE The present invention relates to granular sodium perborate monohydrate having a high bulk density and a method for making it. More particularly, the present invention relates to sodium perborate monohydrate of a high bulk density having an active oxygen content in excess of 16% by weight while retaining other desirable properties that make sodium perborate monohydrate suitable as the source of hydrogen peroxide in bleaching operations in laundry detergents.

The main industrial application for sodium perborates is as bleaching agents in detergent formulations. They are important components and may account for up to 15% by weight of the final formulation. Both sodium perborate tetrahydrate and sodium perborate monohydrate are currently commercially available and used in the detergents industry.

Developments in the detergents industry have shown a trend to more dense formulations known as compact and super compact products. These products generally have bulk densities at or around 0.9g/cc.

Sodium perborate monohydrate is often favoured over sodium perborate tetrahydrate due to its high percentage active oxygen content and rapid dissolution rate in the laundry wash. Typically, a 1 g sample will dissolve in 500 ml of water at 15 C in less than one minute. However, commercially available sodium perborate monohydrate has a bulk density of around only 0.6 g/cc. It would therefore be advantageous to have a sodium perborate monohydrate with a bulk density approaching that of compact washing powders while retaining its other desirable properties.

Sodium perborate monohydrate is commercially produced by the thermal dehydration of sodium perborate tetrahydrate. Under the processing conditions, the morphology of the sodium perborate tetrahydrate is not affected but the loss of water from the granules results in a product having a highly porous structure with an associated reduction in bulk density. Typical properties are an active oxygen content of 16% by weight, a bulk density of around 0.6 g/cc and a dissolution rate in water, as above, of less than one minute. However the product has a low attrition resistance which leads to copious amounts of fine material being produced during manufacture (typically 5-10%). These fines are usually recycled to the process for making sodium perborate tetrahydrate.

It has been proposed to obtain sodium perborate monohydrate of improved attrition resistance.

Thus GB-A-1520127 describes a process in which sodium perborate tetrahydrate particles are contacted with a warm inert gas under specified conditions for removal of at least 0.8 moles of water of crystallisation.

US-A-4002434 describes a process for producing abrasion resistant sodium perborate monohydrate by addition of water, e.g. by spraying, in a moistening unit to preformed sodium perborate monohydrate and then drying under specified temperature conditions. The water used may contain 5-10% by weight of a water- soluble material. The use of a 2% hydrogen peroxide solution is exemplified. US-A-4942025 describes a process for preparing granular, abrasion resistant, sodium perborate monohydrate in which, prior to its dehydration, a carboxylic acid or salt thereof is added to the starting granular sodium perborate tetrahydrate.

It has also been proposed to make granular sodium perborate monohydrate having a bulk density in excess of 0.6 g/cc.

US-A-5094827 discloses the continuous production of sodium perborate monohydrate granulates by granulating primary particles produced by spraying an aqueous solution containing hydrogen peroxide and sodium metaborate into a hot gas and drying. The sodium perborate primary particles are granulated in the presence of a granulating agent which may be water or sodium perborate tetrahydrate. Hydrogen peroxide<I>inter</I> alia is mentioned as a possible additive to the water.

US-A-5395602 describes a compaction method whereby sodium perborate monohydrate having a low bulk density is compacted between rollers at a contact pressure of 5x103 to 2x104 N/cm. The shells so produced are comminuted and sieved according to particle size.

The sodium perborate monohydrate particles obtained according to the methods of US-A-5094827 and US-A-5395602 have active oxygen contents under 16% by weight.

The present invention concerns the obtaining of sodium perborate of increased bulk density by agglomeration using concentrated hydrogen peroxide solution as agglomerating agent.

Further, it has surprisingly been found that, when concentrated hydrogen peroxide solutions are used in the agglomeration of sodium perborate monohydrate, the active oxygen content of the agglomerated particles is increased over that of the fine starting material. It is believed that this increased active oxygen content results from reaction between the sodium perborate monohydrate and hydrogen peroxide forming a perhydroxyl - boron bond instead of the usual hydroxyl - boron bond.

In particular it has surprisingly been found that sodium perborate monohydrate particles of high bulk density and also high active oxygen content may be obtained by agglomeration using a high concentration hydrogen peroxide solution as agglomeration agent. This process may be used for the agglomeration of the fines obtained in the manufacture of sodium perborate monohydrate by dehydration of sodium perborate tetrahydrate. According to a first aspect of the present invention, there is provided a method for producing sodium perborate monohydrate particles of improved, bulk density which method comprises agglomerating sodium perborate monohydrate particles using an agglomerating agent comprising at least 20%, preferably at least 30%, weight by volume aqueous hydrogen peroxide solution, and then drying the agglomerated particles.

The invention also provides the agglomerated particles obtained by the method of the invention.

Suitably the sodium perborate monohydrate particles which are treated are the fines produced as undersized materials in the manufacture of sodium perborate monohydrate by the dehydration of sodium perborate tetrahydrate. Such fines typically have a bulk density in the region of 0.6 g/cc.

The sodium perborate monohydrate particles to be agglomerated typically have a particle size of less than 350 pin. A significant proportion of the starting particles may be below 45 p.m in size. However generally the majority of the starting particles will be in the range 45 to 300gm, more particularly 125 to 300 gm, and preferably 180 to@250p.m.

Typically, following screening, the agglomerates obtained according to the invention are 710 to 90 pin in size. The final agglomerate size will, naturally, depend amongst other things on the starting particle size.

The starting sodium perborate monohydrate particles will normally have a bulk density in the region of 5.5 to 6.5 g/cc. The final agglomerates obtained according to the invention may have a bulk density above 0.7 g/cc, e.g. 0.7 to 0.9 g/cc.

The agglomerating agent used is suitably an aqueous 20 to 80% weight by volume hydrogen peroxide solution. Generally there is used, as agglomerating agent, an aqueous hydrogen peroxide solution having a concentration of at least 30% weight by volume and the upper limit may generally be 70% weight by volume. Preferably there is used, as agglomerating agent, an aqueous 30 to 60% weight by volume hydrogen peroxide solution.

A further advantage of the invention is that the agglomerated particles obtained are generally more resistant to attrition than the irregularly shaped particles of commercially available sodium perborate monohydrate as the effect of agglomeration is to produce particles substantially spherical in shape and having a smooth surface instead of a dendritic particle shape. The spherical particle shape ensures better packing, which together with the reduced porosity of the product, results in a higher bulk density over sodium perborate monohydrate manufactured by conventional means.

According to the present invention, sodium perborate monohydrate particles in a finely divided state produced as undersized material in the manufacture of sodium perborate monohydrate by the dehydration of sodium perborate tetrahydrate and having a bulk density of about 0.6g/cc, may in particular be agglomerated to increase their bulk density in the presence of an agglomerating agent comprising at least 30% weight by volume aqueous hydrogen peroxide solution followed by drying.

Further, according to the invention, particles of fine sodium perborate tetrahydrate are manufactured by means known to those skilled in the art, e.g. by adjusting the crystallisation conditions to obtain fine particles having a particle size generally less than 350 gm. These are then partially or fully dehydrated and may be agglomerated to increase their bulk density in the presence of an agglomerating agent comprising at least 30% weight by volume aqueous hydrogen peroxide solution followed by drying.

Additionally, as indicated above, there may be obtained by the process of the present invention sodium perborate monohydrate particles of increased active oxygen content. Typically the starting sodium perborate monohydrate particles may have an oxygen content of 15 to below 16% by weight, while the agglomerated product will have an active oxygen content of at least 16 % by weight to 18% by weight.

Thus, sodium perborate monohydrate having a particle size less than 350pm and an active oxygen content of 15.5% by weight, granulated by spraying with 30% weight by volume hydrogen peroxide solution and oven dried, was found to have an increased bulk density to 0.77 glcc due to better packaging and reduced porosity, and an active oxygen content of 16.7% by weight.

To maximise the benefit of the invention by increasing the overall active oxygen content of the sodium perborate monohydrate, an agglomerating agent containing up to 60% weight by volume of hydrogen peroxide may be used. Amounts below 30% do not generally significantly affect the overall increase in bulk density of the sodium perborate monohydrate nor the particle shape and the effects of improvements in active oxygen content are diminished.

The agglomeration according to the invention may be carried out in a conventional manner by adding the hydrogen peroxide in the form of a fine spray.

Techniques for the agglomeration of small particles to provide larger particle size products are well known. A comprehensive review of processes for agglomeration and which can be used in the process of the present invention, is given in Kirk Othmer's Encyclopedia of Chemical Technology, 4' Edition (22) pp 222-255 The granulating device used may be any apparatus suitable for the mixing of dry particulate materials adapted so that a liquid granulating agent can be sprayed on or otherwise added to the particles. Conventional granulation equipment can be used. An intensive mixer, such as an Eirich mixer, may be used.

By proper adjustment of the process variables, such as the rate and amount of hydrogen peroxide solution added, speed of rotation of the mixer and placement of spray to achieve maximum contact of the hydrogen peroxide solution with the sodium perborate monohydrate particles, the desired agglomerated product is obtained. Undersize and/or oversize particles can be removed, for instance by screening, if a narrow particle size distribution is desired for the agglomerated product. The oversize can be crushed and recycled.

The time required for agglomeration is dependent on the rate at which the hydrogen peroxide solution is added and the speed of agitation. A suitable agglomeration time is about 15 minutes.

The hydrogen peroxide solution is suitably used in an amount of about 14% by weight based on the total weight of the particles. It is sprayed through fine jets directly onto the material being mixed in a granulating device at ambient temperature of about 15 C to 25 C. The droplet size is suitably about 50 microns.

After the agglomeration step, the product is dried. Drying is conveniently carried out in a fluid bed drier operating at about 50 C with a typical residence time of about 30 minutes.

The sodium perborate monohydrate granulated product prepared in accordance with the invention dissolves very rapidly in water. The rate of dissolution is consistent with that desired for products being incorporated into detergent formulations.

The active oxygen content of the sodium perborate monohydrate according to the invention is significantly higher than that of the starting material and has been found to be stable on standing in the open at ambient temperature for six weeks.

It is possible according to the invention to obtain sodium perborate monohydrate particles of high bulk density and an active oxygen content above 16% by weight. Accordingly the invention provides sodium perborate monohydrate particles having a bulk density of at least 0.7 g/cc, e.g. 0.7 to 0.9 g/cc, and an active oxygen content of 16 to 18% by weight, e.g.<B>16.7</B> to 17.8 % by weight.

The attrition index of the sodium perborate monohydrate according to the invention according to ISO Test 5937 is generally less than 10% and the bulk density above 0.7 g/cc.

The invention further provides detergent compositions containing sodium perborate monohydrate particles according to the invention and the use of those particles in detergent compositions.

The invention is further illustrated by reference to the following Examples. EXAWLE 1 Sodium perborate monohydrate fines (3000g) having a particle size less than 350Etm was charged into an Eirich mixer and agitated at 1300 rpm. 30% weight by volume hydrogen peroxide solution (420g) was sprayed directly onto the fines through an ultrasonic sprayer at a rate of 30 ml per minute.

After all the hydrogen peroxide had been added, the mixer was stopped and the product was transferred to a fluid bed dryer operating at 50 C for a period of 30 minutes.

The product was a free-flowing granular material having the appearance of sodium perborate monohydrate manufactured by conventional means. On microscopic examination, the individual particles were seen to have smooth surfaces. The active oxygen by weight was measured at 16.7 and the bulk density was 0.77 g/cc.

EXAMPLE 2 The method of Example 1 was followed except that 60% weight by volume hydrogen peroxide solution was used instead of 30%.

The product was similar in appearance to that of Example 1. The % active oxygen by weight was measured at 17.8 and the bulk density was 0.77 g/cc.

Measured by volume, the % active oxygen increases to 12.9 in respect of Example 1 and to 13.7 in respect of Example 2. The starting material had 9.0% active oxygen by volume.

Claims

CLAIMS 1. A method for producing sodium perborate monohydrate particles of improved bulk density, which method comprises agglomerating sodium perborate monohydrate particles using an agglomerating agent comprising at least 20% weight by volume aqueous hydrogen peroxide solution, and then drying the agglomerated particles.
2. A method according to claim I wherein there is used as agglomeration agent 20 to 80% weight by volume aqueous hydrogen peroxide solution.
3. A method according to claim 1 or 2 wherein the starting sodium perborate monohydrate particles have a particle size below 350 [m, a bulk density of 5.5 to 6.5 g/cc and an active oxygen content of 15 to below 16% by weight.
4. A method according to any preceding claim wherein the starting sodium perborate monohydrate particles are undersized material from the manufacture of sodium perborate monohydrate by the dehydration of sodium perborate tetrahydrate.
5. A method of producing sodium perborate monohydrate particles of improved bulk density substantially as described in Example 1 or 2.
6. Agglomerated sodium perborate monohydrate particles produced by the method claim in any preceding claim.
7. Sodium perborate monohydrate particles having a bulk density of at least 0.7 g/cc, and an active oxygen content of more than 16 to 18% by weight.