FR2753110A1 - Dense powder - Google Patents

Abstract

The dense powders of density 350-700 kg/m<3> are obtained by a pneumatic method and contain 10-40% of particles of size <1 mu m.

Description

Dense powder based on silica dust
Field of the invention
The invention relates to the field of silica dust resulting from the filtration of fumes from electric furnaces used for the production of metallurgical silicon, ferrosilicon or silicon alloys. These dusts are used in particular as additives in cements and concrete.

State of the art
The production of silicon, ferrosilicon and silicon alloys in an electric furnace releases large quantities of famées which must be filtered to prevent their release into the atmosphere. The dust resulting from this íiltration consists essentially of fine particles of colloidal silica, with an elementary size of between 0.01 and 0.60 Fm, collected in aggregates with a maximum dimension of between 0.2 and 1 am. This dust has a low density, of the order of 120 to 180 kg / m3, which entails a high cost of transport per unit of weight between the producer and the place of use.

In order to reduce their transport cost, we have therefore been led to resort to various treatments. It was first proposed to form an aqueous paste with silica by adding water. In this way, fairly stable water-silica mixtures containing from 30% to 70% by weight of each of the constituents are obtained. However, transporting a large amount of water is not a very economical solution.

ELKEM's patent FR 2349539 describes a pneumatic densification process, in which the dust is kept in a fluidized state in a hopper by air injected by the base. Patent FR 2349540, also from ELKEM, describes a mechanical densification process using a rotating drum, optionally provided with internal ribs. GRANGES patent FR 2363369 describes a mechanical densification process by passing through a ball or bar mill.

These techniques make it possible, depending on the case, to obtain densities ranging from 300 to 700 kg / m3. They generally have the effect of bringing together the aggregates into agglomerates, the maximum dimension of which is between 1 and 100 Fm.

Unfortunately, applications of silica dust almost all require a fine particle size, and users most often seek to redisperse the agglomerates to find aggregates whose dimensions are as close as possible to those of the aggregates of the original non-densified product. To do this, dispersing products are added to the densified dust, before use, as indicated for example in patents FR 2537127 (ELKEM) or FR 2693128 (AXIM).

However, experience shows that this redispersion is not easy and that the aggregates obtained are always larger than the original ones. The object of the invention is to obtain a densified powder based on silica dust which, on redispersion, forms aggregates of a size comparable to those of the powder before densification.

Object of the invention
The subject of the invention is a dense powder based on silica dust consisting of a mixture of 70 to 98% (by weight) of densified powder and of 2 to 30% of non-densified powder. The densified powder preferably results from pneumatic densification.

Description of the invention
The invention is based on the surprising finding made by the applicant that by adding up to 30% of non-densified powder to an already densified powder, a mixture is obtained, the density of which clearly exceeds, sometimes by more than 30%, the theoretical density, i.e. that resulting from the formula: l / d = xl / dl + x2 / d2 where x1 and x2 represent the mass fractions and dl and d2 the original densities of the densified and non-densified dust .

However, examination by microscopy of such mixtures shows that the added non-densified dust is indeed found in the final mixture in the form of unmodified aggregates.

This non-densified dust thus constitutes an additional supply of small-sized aggregates (0.2 to 1 clam), which no longer has the drawback of low density.

The difference between the actual density and the theoretical density decreases markedly when the quantity of non-densified dust introduced into the densified dust exceeds 30%. Thus, when less than 30% of densified dust is added to non-densified dust, the difference is practically non-existent; surprisingly, the difference is greater if the dust has been densified by a pneumatic process, i.e. 'That is to say by injecting air into the dust maintained in the fluidized state.

The invention makes it possible to obtain both powders with a density of between 350 and 700 kg / m3, allowing transport under acceptable economic conditions, and comprising a level of particles of less than 1 ssm of between 10 and 40%, which represents a very substantial improvement in the fineness of the particle size compared to the densified powders of the prior art, which facilitates redispersion in the most common applications, in particular the addition in concrete.

Examples
Example 1
From a silica dust of density d1 = 130 kg / m 3, a pneumatically densified powder of density d2 = 690 kg / m 3 is prepared. 10%, 20% and 30% of the starting non-densified powder are then mixed with this densified powder, respectively, and the theoretical and actual volume masses of the 3 mixtures are compared.

The results are given in Table 1.

Table 1

<tb> Addition <SEP> d <SEP> theoretical <SEP> d <SEP> real <SEP> Difference
<tb><SEP> 10% <SEP> 480 <SEP> 600 <SEP> + <SEP> 25% <SEP>
<tb><SEP> 20% <SEP> 370 <SEP> 490 <SEP> + <SEP> 33% <SEP>
<tb><SEP> 30% <SEP> 300 <SEP> 350 <SEP> + <SEP> 16%
<tb>
It can be seen that the maximum difference is obtained with an addition of 20% of non-densified powder.

By laser particle size measurement, it is found that 98% of the grains of the non-densified powder have a size of less than 2 μm, while only 3% of the grains of the densified powder have a size of less than 2 μm. On the 3 mixtures, it can be verified that the actual percentage of grains of size <2 zm is practically identical to the theoretical percentage, as shown in Table 2.

Table 2

<tb>% <SEP> not <SEP> densified <SEP>% <SEP><<SEP> 2 <SEP> lim <SEP> theoretical <SEP>% <SEP><<SEP> 2 <SEP>, um <SEP> real <SEP>
<tb><SEP> 10 <SEP> 12.5 <SEP> 12
<tb><SEP> 20 <SEP> 22.0 <SEP> 21.5
<tb><SEP> 30 <SEP> 31.8 <SEP> 31
<tb>
Example 2
The starting point is a non-densified powder of density d2 = 200 kg / m 3, part of which is densified by a pneumatic fluidized bed process to a density of 670 kg / m 3. Various mixtures of the starting powder and the densified powder are prepared at different levels and the actual density is compared to the theoretical density. The results are shown in Table 3.

Table 3

<tb> no <SEP> densified <SEP> d <SEP> theoretical <SEP> d <SEP> real <SEP>% <SEP> difference
<tb><SEP> O <SEP> 670 <SEP> 670 <SEP> 0
<tb><SEP> 10 <SEP> 542 <SEP> 620 <SEP> 14
<tb><SEP> 20 <SEP> 456 <SEP> 560 <SEP> 23
<tb><SEP> 30 <SEP> 393 <SEP> 465 <SEP> 18
<tb><SEP> 80 <SEP> 233 <SEP> 235 <SEP> 0.5
<tb>

<tb> 90 <SEP> 215 <SEP> 1 <SEP> 215 <SEP> 1 <SEP>
<tb> 95 <SEP> 208 <SEP> 208 <SEP> o <SEP>
<tb> 100 <SEP> 200 <SEP> 1 <SEP> 200 <SEP> 1
<tb>
It is observed that there is practically no difference between the theoretical density and the density measured for the high contents of non-densified powder.

By laser granulometry, a rate of grains of size less than 1 m of 95% on the non-densified powder and of 1% on the densified powder is measured. We check
also on this example that, for the different mixtures, the actual rate of
size less than 1 m is very close to the theoretical rate, which confirms that the fine
particle size of the non-densified powder is maintained in the mixture. The results
are shown in Table 4.

Table 4

<tb>% <SEP> not <SEP> densified <SEP>% <lllmihéorique <SEP>% <SEP><<SEP> 1 <SEP> m <SEP> real <SEP>
<tb><SEP> 10 <SEP> 10.4 <SEP> 10
<tb><SEP> 20 <SEP> 19.8 <SEP> 20
<tb><SEP> 30 <SEP> 29.2 <SEP> 29
<tb><SEP> 80 <SEP> 76.2 <SEP> 75
<tb><SEP> 90 <SEP> 85.6 <SEP> 85
<tb><SEP> 95 <SEP> 90.3 <SEP> 90
<tb>

Claims (4)

by weight of densified dust and 2 to 30% of non-densified dust. CLAIMS 1. Dense powder based on silica dust consisting of a mixture of 70 to 98% 2. Dense powder according to claim 1, characterized in that the dust were densified by a pneumatic process. 3. Dense powder according to one of claims 1 or 2, characterized in that its density is between 350 and 700 kg / m3. 4. Dense powder according to one of claims 1 to 3, characterized in that it contains from 10 to 40% of particles of size <1, eLm.