HK1065993B - Abrasion-resistant agglomerate mineral substance granule, powder comprising such granules and production method therefor - Google Patents

Description

Wear resistant agglomerated mineral particles, powder containing same and method for making same

The present invention relates to attrition resistant agglomerated mineral particles (grain de substance minutiae). The invention also relates to a powder formed from the granules, to the use thereof and to a method for producing the same.

Many minerals, such as sodium and magnesium salts, are commonly used in particulate form.

In order to use these particles effectively, generally the particles must have a suitable particle size. For example, for many applications, such as particles for use as detergents, pharmaceuticals or pharmaceutical additives, the desired average particle size is 250-1000 μm.

An important indicator in the particle production process is its particle size distribution. Some methods result in a significantly narrow distribution. Unfortunately, their cost is often high. The particle size distribution of particles produced by the less expensive process is generally very broad and therefore the particle size distribution of all particles produced is hardly satisfactory for a particular application. In these cases, it is known to classify the particles according to their size, for example by sieving, in order to adapt them to different applications.

In order to utilize all the particles produced, the consumption of the various grades of particles must be adapted to the production capacity. This is a rare case and is generally the case with much greater amounts of small particles than large particles.

In addition, when the crystallizer is used to produce granules, the yield (in tons/hour) is significantly reduced when large granules need to be produced. And the production cost is obviously increased.

To address this problem, it is known to agglomerate (agglomerer) small particles of the mineral to give larger particles. A method that can be used is a mechanical agglomeration method, such as pressing. However, these agglomerated products have the disadvantage of being brittle. Since these particles have poor abrasion resistance, fine particles are again present when they are handled. In addition, the known agglomeration methods are only applicable to minerals that are easily agglomerated. For other particles, the known methods are difficult to use because these known particles are fragile.

To overcome this drawback, attempts have been made to add anionic surface additives during the manufacture of the agglomerates. Such use is described, for example, in EP0452164, which relates to sodium perborate tetrahydrate of improved attrition resistance formed from an agglomerate of isolated particles. The size of the particles does not exceed 30 μm. However, the addition of additives has certain drawbacks for the manufacture of mineral particles when high purity mineral particles are to be produced. Such high purity products are particularly desirable in pharmaceutical processes.

The object of the present invention is therefore to overcome these drawbacks by providing granules of agglomerated minerals which have good wear resistance and which can be manufactured by a simple, economical process without the use of any additives.

The invention therefore relates to a granulate comprising agglomerates of mineral granulate, characterized in that the agglomerates are covered with an integral outer layer.

The particles of the present invention may have any shape. It may have a smaller or larger particle size. The economic benefit of the present invention will be more pronounced in the case of large particle diameters. This is because the cost of the production process of the present invention increases with particle size to a lesser extent than in the prior art.

The particles according to the invention have the advantage that their average diameter is 200-5000 μm, preferably greater than 250. mu.m. Preferably, the diameter is less than 2500 μm. When the particles are approximately spherical, the average diameter is its actual diameter. When the particles have an arbitrary shape, the average particle diameter thereof is defined as six times the ratio of the volume to the external surface area.

The invention is applicable to any mineral where it is desirable to obtain large particles from smaller particles. Such as sodium or magnesium chloride, sodium carbonate, sodium bicarbonate. The invention has the advantages that: it is suitable for agglomerating mineral particles with poor mechanical properties.

The invention is particularly applicable to sodium bicarbonate.

The number of agglomerates comprising particles is determined by the average diameter of these particles and the size of the particles they form, and can range from tens to millions.

According to the invention, the agglomerates are coated with an integral outer layer. The term "integral outer layer" refers to a layer of substantially continuous material. The material is not obtained by agglomeration. The material used to prepare the outer layer is essentially a single, non-agglomerated mass. It is preferably crystalline. The layer may be monocrystalline. More often, it is polycrystalline. The monolithic layer forms an outer shell on the agglomerate surface, the function of which is to give mechanical protection to the particles. This layer may be impermeable or porous or have some defects, but it is sufficient if it has the above-mentioned functions. The bulk layer covering the agglomerates may also penetrate deeper into the agglomerates at certain locations by penetration of the cracks that were originally present in the agglomerates.

The thickness of the coating layer should be sufficient to fulfill its technical function as described above and to provide the desired good mechanical properties. But its thickness does not have to be too large because it would reduce the economic efficiency of the present invention.

The minimum thickness of the integral outer layer is determined by various parameters, in particular the substance forming the integral outer layer, the size of the agglomerates, the average particle size of the particles forming the agglomerates and the required mechanical strength. In general, the thickness must be determined by routine laboratory studies in each particular case.

The overall outer layer thickness is typically greater than 0.25 μm. It is generally advantageous for the layer thickness not to exceed 50 μm. Thicknesses of more than 0.5 μm are particularly suitable. Preferably, its thickness is less than or equal to 30 μm.

According to the invention, coating the agglomerates with an integral outer layer significantly improves their wear resistance.

The choice of overcoat material is determined by the final desired properties. In addition to abrasion resistance, the coating layer using, for example, an organic material can enhance antistatic property and water resistance.

However, it is advantageous to form the integral outer layer from a mineral.

Preferably, the mineral of the integral outer layer is substantially the same as the mineral of the granules. An advantage of this embodiment is that particles of high purity can be obtained.

In a preferred embodiment of the invention, the particles do not contain additives such as binders or surfactants. In a preferred form of embodiment, the particles are free of binder. Such particles are able to meet the most stringent purity requirements, both in the human food and pharmaceutical fields.

The invention also relates to a powder comprising a plurality of such particles as described above.

The powder of the present invention may be formed of only the particles of the present invention. It may also contain other particles in different proportions.

Since the width of the particle diameter distribution depends mainly on the agglomeration process used, it can vary greatly.

The powder of the invention can be used in different technical fields. In particular, the powder of the invention containing sodium bicarbonate particles can be applied to the manufacture of detergents or pharmaceutical agents.

The invention also relates to a method for producing said powder, wherein, in a first step, agglomerates of mineral particles are formed; the method is characterized in that in a second step, the agglomerate is coated with a monolithic layer.

The effect of the first step of the process of the invention is to agglomerate particles having an average diameter that is too small for practical use to form agglomerates having an average diameter suitable for use in these applications. Any conventional agglomeration method may be used, for example, sintering, stacking or pressing.

As mentioned above, the mineral particles agglomerate to a greater or lesser extent with respect to each other, depending on the mineral from which the agglomerates are formed and the method used to agglomerate them. However, to ensure that the agglomerates are bonded before they are coated, a minimum amount of adhesion is necessary.

In one embodiment of the invention, the agglomerates of particles are formed by pressing. Any suitable pressing method may be used. In an advantageous way of carrying out the process, the granules are compacted by compression between two rollers, and the resulting solid mass is then comminuted in order to obtain agglomerates of the desired size. The resulting product is sieved and crushed and agglomerates of too small a particle size are pressed again.

In the second step of the process of the invention, the agglomerate is coated with an integral outer layer. The coating method used, for example dusting, spraying or dipping, is selected according to the particular circumstances applicable.

In one advantageous form of the method of the invention, a supersaturated solution of the bulk layer composition is passed through the bed of agglomerates to coat the agglomerates. The bed of agglomerates may be fluidized or non-fluidized. A fluidized agglomerate bed is preferred. The growth of crystals by flowing a supersaturated solution through a fluidized bed has been described in EP0352847(SOLVAY SA). To use the method in the present invention, a fluidized bed is formed with the agglomerates to be coated. While a supersaturated solution may be obtained by cooling a saturated solution before contacting the flowing layer.

In embodiments where the integral outer layer and agglomerates are made with sodium bicarbonate, the temperature of the fluidized bed is advantageously above 30 ℃. However, a temperature of 70 ℃ or higher is not worth.

In a preferred method of achieving this, the temperature of the fluidised bed is above 40 ℃ and below 60 ℃.

In another advantageous mode of the method according to the invention, a supersaturated solution of the overall skin composition is sprayed onto the agglomerates in order to coat them. In a preferred method of accomplishing this, the agglomerates are continuously introduced into an inclined rotating drum that is sprayed with a supersaturated solution. The movement of the drum ensures that the entire outer layer is evenly distributed over the agglomerates. The coated agglomerates can be removed from the drum over a sufficiently long period of time, which is determined in particular by the desired coating thickness and can be adjusted by the inclination, size and speed of rotation of the drum.

The invention is illustrated by the following description with reference to the accompanying drawings.

This figure illustrates a schematic apparatus of one embodiment of the process of the present invention.

The schematically illustrated apparatus comprises a roller press 2, a pulverizer 4, a screen frame 6, a wetting tank 8, a heat exchanger 10, a fluidized bed crystallizer 12 and a dryer 14, the operation of which will be described below.

Sodium bicarbonate particles 1 are added to a roller press 2. In which a press cake 3 is formed. The pressed cake 3 is then comminuted in a comminuting machine 4 to form agglomerates 5. A fraction 7 having a particle size of 500-1000 μm is sieved from the agglomerate 5 through a sieve frame 6. The undersize 16 of less than 500 μm is recycled to the roller press 2, while the oversize 17 of more than 1000 μm is recycled to the pulverizer 4. The selected agglomerates 7 are fed to a crystallizer 12, which forms a fluidized bed with said agglomerates. A saturated sodium bicarbonate solution 9 is formed in the bath 8. The solution is cooled in a heat exchanger 10 to produce a supersaturated sodium bicarbonate solution 11. The supersaturated solution 11 is fed to a crystalliser 12 where a bed of crystals is fluidised. Solution 11 is desaturated after contact with the crystals, thereby gradually covering the crystals with an integral layer of sodium bicarbonate. The wet, coated bicarbonate 13 collected from the crystallizer 12 is dried in a dryer 14 to obtain the final product 15.

In the preferred embodiment of the apparatus shown in the figure, the infiltration tank 8, the exchanger 10 and the crystallizer 12 are combined into one unit. Namely the device described in EP0352847(SOLVAY SA).

The effects of the present invention will be explained in conjunction with the embodiments described below.

Example 1 (not according to the invention)

The following mechanical milling test was performed on pressed and crushed sodium bicarbonate agglomerates having a particle size of 500-.

100g of sodium bicarbonate particles are added in the presence of 400g of lead beads having a diameter of 6mm in a cylindrical drum having an internal diameter of 57mm and a length of 120 mm. The drum was then rotated at 140rpm for 30 minutes. After this treatment, the particles were tested for "attrition index", which is defined as the weight percent of particles in the drum that pass through a 63 μm sieve. The result of this test was 3%.

Example 2 (inventive)

100g of a fluidized bed of pressed and crushed sodium bicarbonate agglomerates having a particle size of 500-1000 μm were placed on a fluidization grid of a fluidization column connected to a thermostatic bath set at a temperature of 50 ℃. The bed of agglomerates was fluidized as the supersaturated sodium bicarbonate solution rose through the flow holes at a rate of 154 m/h. The supersaturation of the supersaturated sodium bicarbonate solution was 2.4g/kg, since the temperature dropped by 1.9 ℃ as the solution flowed through the heat exchanger.

The agglomerate was coated for 1 hour, after which the particles were collected and the average thickness of the coating layer was measured to be 3 μm. The granules according to the invention prepared in this way were subjected to the attrition resistance test as defined in example 1 above, with the result that they were 0.6%. The coating layer of the invention can obviously improve the wear resistance of the particles.

Example 3 (inventive)

The procedure of this example was the same as in example 2 except that the supersaturation degree of the supersaturated sodium bicarbonate solution was set to 4.9g/kg, after which a 10 μm coating was deposited on the agglomerates. The abrasion resistance test result was 0.1%. This in turn demonstrates the improved attrition resistance of the particles of the present invention.

Claims (10)

1. A granule comprising agglomerates of mineral particles, characterised in that the agglomerates are coated with an integral outer layer formed of a mineral, wherein the mineral of the particles and/or the mineral of the integral outer layer is selected from sodium or magnesium salts.

2. Granulate according to claim 1, characterised in that the mineral of the granulate or of the integral outer layer is selected from sodium or magnesium chloride, sodium carbonate and sodium bicarbonate.

3. A granule according to claim 1 or 2, characterized in that the mineral of the integral outer layer is the same as the mineral of the granules.

4. Granulate according to claim 1 or 2, characterised in that the mineral of the particles and/or of the integral outer layer is sodium bicarbonate.

5. Granulate according to claim 1 or 2, characterised in that it is free of binder.

6. A particle according to claim 1 or 2, characterised in that the thickness of the integral outer layer is 0.25-50 μm.

7. A powder composition comprising the particles of any one of claims 1-6.

8. A method of manufacturing a powder composition according to claim 7, wherein in a first step agglomerates of mineral particles are formed, characterized in that in a second step the agglomerates are coated with an integral outer layer.

9. The method of claim 8, wherein the particulate agglomerates are formed by pressing.

10. A method according to claim 8 or 9, characterized in that the agglomerates are coated by flowing a supersaturated solution of the bulk layer constituent material through the bed of agglomerates.