Process for surface treatment of particles of micronised materials

The present invention relates to a particle surface treatment method of finely micronized powder materials.

Many industries including the plastics industry use of mineral fillers which are mixed with the base plastic. It is in particular known to use finely ground calcium carbonate whose particles have previously undergone a surface treatment with fatty acid mixed or not with fatty acid esters.

These fine powders of calcium carbonate have, usually, an average size of 1 to 5 · 10⁻⁶ m, with an upper limit of 6 to 25 · 10⁻⁶m.

Treating the surface of these powders is to coat the grains of the fatty acid (alone or in admixture with fatty acid esters), in a medium or are:<ul><li>- The relatively high temperature liquid fatty acid,</li><li>- The well dispersed grains in the hot air in vortical motion,</li></ul> This is to ensure an intimate and homogeneous absorption of the liquid fatty acid with calcium carbonate grains, with a partial chemical reaction between the materials in contact, producing a thin layer of calcium stearate on the surface of seeds, the Materiality depends on the physical conditions of treatment and the nature of the raw material.

Therefore the known methods comprise proceed initially to dry, then grind and then classification of the raw material to obtain a dry powder having the desired final particle size, and then proceeding to the coating and surface treatment of the powder in a second time.

This requires having several facilities, including:<ul><li>1) grinding drying plant (possibly with embedded classification);</li><li>2) gross grinding powder classification installation;</li><li>3) coating installation - surface treatment of the final powder; which makes the known processes, lengthy, and expensive.</li></ul>

Known from FR-A 1562280 a plant in which simultaneously performs dehumidification and reduction of fine powder clay clods and mix of organic coating material by passing this mixture together with a gas such as hot air through a disintegrator capable of reducing to a fine powder.

However, it turns out that this facility does not provide a sufficiently fine calcium carbonate powder, and this especially because the disintegrator permits to split the carbonate in virtually particle size greater than 25 microns. The only particles that might be likely to be acceptable are contained in the air escaping from the cyclone and are lost.

Known also by the FR-A-1583182 patent a process for preparing water-repellent talc powder. The water repellent is mixed with the talc in the raw state, that is to say coarse powder, the mixture is placed in a fluidized bed state at a temperature close or preferably above the melting temperature of the water repellent (paraffin) and is subjected to a pneumatic grinding treatment in a micronizer rotors.

However, this process and its installation have significant drawbacks:<ul><li>- The realization of a prior fluidized bed coating treatment itself requires grinding powder beforehand (even gross) the ore; and to have the flow and temperature control devices, which significantly raises the cost of installation,</li><li>- The thinnest dimension obtained is not less than 10 microns, which is very insufficient for use in PVC.</li></ul>

In addition other characteristics of coated carbonate particles is their moisture content. It should not be less than 0.5%; and the preferred level is of the order of 0, 1%. This requirement is necessary to get a good flow of particles and prevent agglomeration, which would make possible their use in PVC.

The present invention aims to provide an improvement to the devices and methods described in these previous installations, which gives in one treatment a powder of calcium carbonate particles with dimensions between 1 and 25 · 10⁻⁶ m, coated calcium stearate and stearic acid, having a maximum moisture content of about 0.5%.

The invention relates to a continuous process of coating calcium carbonate by means of a fatty acid of the stearic acid type, of the type which comprises mixing the wet calcium carbonate and préconcassé with the solid fatty acid in the form of pieces are introduced the wet mixture in a mill through which a heated air stream in a heating device, characterized in that:<ul><li>- The carbonate grains and the pieces of acid are of dimensions less than 1 cm;</li><li>- Grinding is an ultra-fine micronisation;</li><li>- The hot air is introduced moving tourbillonnair e in the micronizer, to an elevated temperature of about 70 ° C to 120 ° C at least, the hot air having the function of mixing and homogenizing the mixture, liquefy the fatty acid, and dries very quickly carbonate, such that the carbonate grains are firstly ground and dried, and secondly powdered increasingly finely and brought into internal contact with the liquid fatty acid, and homogenized; and in that:</li><li>- A portion of the air is extracted from the microniser and recirculated in the heating device while the other part carries the particles first coated in a classification device which separates the heaviest particles and then lead the finest in a storage hopper,</li><li>- The heaviest particles are recycled directly into the moist input mixture in order, first to grind again, and to preheat the mixture and reduce moisture.</li></ul>

The invention also relates to the device for implementing the previous coating process. It comprises a feed hopper for the calcium carbonate, a feed hopper for the fatty acid in the solid state, a centrifugal mill micronizer, a hot air circuit, a classifier particles, a driving recycling, a pipe and a storage hopper, and is characterized in that the heating circuit comprises a recirculation loop, conducting particles recirculation directly feeds the calcium carbonate feed hopper, and in that means control flows are provided (air flow, particles, temperature, hygrometry, pressure) to ensure the operation of the continuous system regardless of the humidity of the mixture, and the constancy of the final product characteristics.

In addition, the device is characterized in that the fatty acid feed hopper discharges into the feed hopper in carbonate by means of a dosing member.

The method of the invention consists in mixing in a micronizing mill the mineral préconcassée from wet quarry in grains of a diameter of the order of 1 cm and the fatty acid, in its solid form, fragmented into pieces having substantially the same dimensions, the mill being passed through by a hot swirling air stream at a temperature between 70 ° C and 120 ° C or lower, depending on the nature of raw material and its moisture content.

The method according to the present invention is implemented in an installation comprising:<ul><li>- A grinder equipped with a fatty acid feed device solidified flowing into the feed hopper of a mineral material, itself flowing through the mill, </li><li>- An air heating device,</li><li>- A static selector large grains, with recycling of these grains in the mill,</li><li>- A dynamic classifier to sort the micronized powder, dried and coated and to recycle the grains whose size is greater than a predetermined limit.</li></ul>

By way of example, to the single figure shows an example of installation for carrying out the method.

Referring to this figure, we see that the installation includes a centrifugal mill 1 of known type, fed by an auger 2 from a hopper 3.

The hopper 3 contains the mineral to be coated or treated surface, which comes from the quarry and was simply pre-crushed without undergoing drying. By "pre-crushed" it is meant that it was coarsely crushed into grains with a diameter of about 1 cm. As it has not been dried, its moisture content can be of the order of 5% to 10% or more.

In the hopper 3, pours a worm 4 from a second hopper 5. Said hopper 5 contains stearic acid, that at normal temperature is in solid form. This stearic acid is also crushed into grains having substantially the same dimensions as the grains of material to be coated. The flow rate of the endless screw 4 is adjustable, and it constitutes a metering mechanism which introduces into the hopper 2 the desired proportion of stearic acid.

At the lower part of the mill 1 there opens a hot air duct 6, from a hot air source 7 where the air is heated to the desired temperature, which is between 70 ° C and 120 ° C. This temperature is determined by the moisture content and hardness of the mineral material in the hopper 3 more this material is wet, the more air must be hot. In the case where the material to be coated is calcium carbonate, it takes almost a hot air temperature of 80 ° when the moisture content is 5%, and 110 ° C when the rate is 10%.

The fatty acid rate introduced into the hopper 3 is dependent on the porosity, absorption capacity of the material and the surface area thereof. Preferably, it is set below the saturation limit of powder micronized so that:<ul><li>- That this powder does not clog after coating, and is fluid enough to be well entrained, dispersed in air and classified correctly to the outlet of the mill,</li><li>- To prevent an excess of fatty acid flowing bottom of disposer.</li></ul>

In the case of stearic acid and calcium carbonate, the rate of fatty acid is from 0.6% to 1.5% of the carbonate.

The installation also includes a static switch placed large grains, as known at the top of the mill classifier 1 and 8 recycling too coarse and sending in a storage hopper 9 micronized powder coated.

Recycling coarse, still hot, is through line 12, directly into the hopper 3 containing the mixture. This arrangement in particular allows to begin the drying of the wet calcium carbonate.

The surface treatment is substantially the same time as the grinding. The hot air dry calcium carbonate, stearic acid liquefies under the centrifugal effect which lines the inner cylindrical wall of the mill. The grains of material, driven by the stream of hot air upward, are violently thrown against the walls, and are broken and micronized gradually while being contacted with the hot liquid fatty acid.

The swirling motion of the updraft in the mill ensures intimate mixing and complete homogenization of micronized calcium carbonate.

A major problem that the invention aims to solve is precisely the drying calcium carbonate. Indeed, it is necessary that the calcium particles are substantially completely dried before being coated, because in contact with stearic acid the ore is formed a calcium stearate layer which prevents moisture remaining in the particle to break free. Drying should be rapid, preferably in the first third of the micronizer.

It is therefore necessary to power the camera with a large volume of hot air. But then, we can not increase the air flow indefinitely because the particles must stay a minimum of time in the jet mill to achieve the desired size.

According to the invention solves this problem by providing a recycling pipe 10 of the hot air in the heating device, either in the air heating, or downstream of the heater at the entrance of the micronizer. Part of the hot air is then recycled, while the other part carries the particles in the classifier then through line 11 to the storage hopper, which makes it possible to have both a large air flow drying and reduced airflow carrier.

On the other hand, in order to control air flows, carbonate and acid, and the moisture contents of the particles, is arranged in the piping of the measuring means (temperature sensors, pressure, humidity , flow meter) and flow control (valve 13).

Experience shows that one realizes at once:<ul><li>- Drying of the mineral,</li><li>- Its grinding to a micronization of dimension of the order of 5 · 10⁻⁶ m,</li><li>- Its surface treatment intimate and regular,</li></ul> so that one gets cheaper a better product.