FR2585691A1 - Process for the preparation of a fine powder of lamellar calcined talc - Google Patents

Abstract

Process for the preparation of a fine powder from lamellar calcined talc consisting a in crushing a talc ore for the purpose of reducing it to a powder, b in agglomerating the said powder to form granules, c in heating the said granules at a temperature between 900 DEG C and 1250 DEG C, and preferably between 1150 DEG C and 1220 DEG C, in order to carry out a complete calcination of the granules, and, after cooling, d in crushing the calcined granules in order to obtain a powder again. The process of the invention makes it possible to produce industrially, an extremely favourable cost, a fine enstatite and amorphous silica powder having a lamellar nature, no longer containing any uncalcined talc whatsoever and completely free of cristobalite.

Description

PROCESS FOR PREPARING FINE POWDER
TALC CALCINE LAMELLAIRE
The invention relates to a process for preparing fine powder of calcined calcined talc. By "talc" is meant either a hydrated magnesium silicate, a hydrated magnesium aluminum silicate, or a mixture of both. Talc cullet means a mixture of enstatite and amorphous silica, obtained after a well known reaction of transformation and recrystallization, called "calcination".

 It is known that finely pulverized talc is used as a reinforcing filler in other materials and in particular in thermoplastic polymeric materials such as polyamides, polyolefins, polycarkonates ... to improve their rigidity and increase their deformation temperature under load. However, these improvements in characteristics are at the expense of the impact resistance of the material, which is very significantly reduced by the addition of talc.

 It has been found that a calcination of the talc, possibly followed by a surface treatment improving the coupling of the filler and the matrix, largely eliminates this defect and makes it possible to obtain a shock resistance of the charged materials satisfactory in a many applications, while largely preserving the improved characteristics usually provided by uncalted talc. Reference may be made to patent FR 82.09524 to Thane Poulenc Specialties Chemicals, which describes a molding composition containing a mineral filler consisting in particular of calcined talc.

 The preparation of this finely pulverized calcined talc thus offers a vital interest in a large number of sectors (automobile industries, electrical and electronic industries, household appliances, etc.). The calcination process described in the above-mentioned FR patent consists in introducing the powder to be calcined in an oven at a temperature of about 1 ° C. for a few hours. However, the specific properties of talc (very high thermal insulating power) ) and the specific properties of the mixture of enstatite and silica obtained during calcination (thermal insulation still increased, strong tendency to sticking and sintering), make it extremely difficult to industrial implementation of such calcination, which regardless of the type of oven used among the various types known for this type of treatment (static oven, rotary kiln, fluidized bed furnace, "flash furnace").

 Experiments have shown that the use in a static furnace is industrially unsuitable because the calcination of the total mass of talc powder can only be done in very thin layers (involving very weak dehits) and requires long periods of time. treatment (at least 4 hours in very thin layer, less than 5 cm thick); if an attempt is made to increase the thickness of the layer in order to obtain economically more satisfactory operating conditions (flow rates, energy consumption), there is either an uncomplete calcination of the powder in the core of the layer or superficial superheating. with cristobalite formation (recrystallized silica): in both cases, the product obtained is unsuitable for use as a filler because it does not provide the desired impact resistance. By way of illustration, an implementation in a layer of 15 cm thick did not allow after 48 hours of heating at 10500 C, calciner than 75% by weight of the talc.

 In rotary kiln with direct flame, there are insurmountable difficulties to maintain the talcum powder in the heating zone for a time suitable for calcination; a part of the products is removed before complete calcination in the gas stream, while another part become sticky and flows very irregularly and is subjected to calcination very heterogeneous in time which leads to either a production of cristobalite or incomplete calcination of the talcum powder. In rotary kiln with indirect heating, the product sticks against the heating walls of the oven and blocks heat transfer; after a short period of operation, it is found that the talc flows out of the oven without being calcined.

 Fluidized bed furnace tests gave negative results and showed that from 2000 to 3000 ° C the talc powder could no longer be fluidized even using mechanically agitated fluidization techniques; above these temperatures, the fluidization becomes indeed unstable and the functionenent equivalent to a fixed bed operation with the above-mentioned faults static furnaces.

 Finally, the tests carried out in existing "flash ovens" have also given negative results with the talc powder, because the very design of these furnaces involves very short contact times between hot gases and products (of the order of half a second to 5 seconds): these times require very high gas temperatures to obtain the complete calcination (at least 1 2000 C) and at these temperatures, the talc powder has a sticky power which causes a very rapid clogging of the installation. For example at 1 2000 C in a flash oven type "GatxrFuller", the calcination process had to be stopped after 75 minutes, because the temperature control was no longer possible due to clogging, and this, whatever In addition, conventional cyclone recovery is not possible and it would be necessary to provide very expensive filtration facilities.

According to the X-ray diffraction measurements which have been carried out, it seems moreover that this rapid calcination process is poorly adapted and produces insufficient crystallization of the enstatite: the tests for introducing talcs thus calcined in Polyamides or polyolefins have not been able to achieve an increase in impact resistance, caused by a suitably calcined talc (probably because of a small percentage of uncalcined talc, inevitahle in practice).

 As a result, one skilled in the art can now obtain in the laboratory a fine layered calcined talc powder having the characteristics suitable for application as filler: talc completely converted to enstatite and amorphous silica, absence of cristobalite detectable by X-ray diffraction (this being obtained in the laboratory by several passages in thin layer in a static oven), but no calcihation specialist has been able to implement this implementation on an industrial scale, so that In practice, there are no fine laminated calcined talc talc powders on the market that meet the requirements of improved impact strength plastics.

 The present invention proposes to remedy this absence by indicating a new process for the preparation of fine powder of calcined calcined talc, which lends itself to an industrial implementation under economically satisfactory conditions.

 The essential objective of the invention is to allow industrial production of a fine powder of enstatite and amorphous silica having a lamellar character, strictly containing no more calcined talc and completely free of cristobalite (precision X-ray analysis ). .

 Another object of the invention is to provide a method that can be implemented from current materials available on the market.

 Another objective is to limit the losses of raw material during the treatment.

 Another objective is to limit pollutions that may affect the calcined talc at different stages of treatment.

 Another objective is to indicate a process benefiting from non-prohibitive energy short.

For this purpose, the preparation method according to the invention consists in combining the following operations
(a) milling a talcous ore to reduce it to a powder having a cutoff diameter D99 of less than approximately 200 microns,
(b) agglomerating said powder to form granules of particle size such that more than 70% by weight of the particles are larger than 500 microns,
(c) heating said granules at elevated temperature between 9000 C and 1 2500 C, so as to bring the granules to this temperature,
(d) after cooling, grinding the calcined granules to obtain again a powder having a cutoff diameter D99 of less than approximately 200 microns.

 By "particles of dimension greater than a given value" is meant particles arrested by a sieve whose mesh is equal to this value; "Cutoff dianeter D99" means the mesh of the sieve giving a refusal percentage equal to 1 S of the total weight of the initial sample, that is to say passing 99 nt by weight of said sample.

 Thus, the process according to the invention paradoxically leads to the preparation of a talcum powder (a) and then destroys it (b) in order to carry out the calcination (c) under good conditions, and finally to restore the said powder (d). ) after this operation. It has been found that this process allows easy and complete calcination of the granules in most types of existing furnace (including rotary kilns), while avoiding the recrystallization of the amorphous silica obtained (absence of cristobalite); at the end of the final regrinding operation (d), a thin, unpolluted fine powder is recovered consisting of a mixture of enstatite and amorphous silica. The final grinding operation (d) is easy to use and conventional, without significant releases, despite the abrasive character and high hardness of enstatite and amorphous silica obtained after calcination (c); moreover, the experiments made it possible to note that this operation preserved the lamellar character of the initial product, which, a priori, is not obvious taking into account the transformation intervened and the recrystallization in enstatite.

 It should be noted that tests have shown that it is not possible to get rid of operations (a) and (b) by direct calcination of raw talc grains from ore prepared to obtain particle size distribution identical to that of agglomerated powder granules mentioned above.In fact, on the one hand, the calcination operation (c) is much more delicate in particular rotary kiln (due to a less favorable behavior of the talus d flow), on the other hand, the final milling operation becomes industrially impractical because of the wear of the mills, the large percentage of discharges obtained (more than 50 b of irreducible particles) and the very low milling rate; moreover, the powder obtained is polluted by metal filings torn off during the final grinding and has a penalizing gray color in applications as a filler. It also seems, according to the experiments carried out by the inventors, that the direct calcination of raw grains does not completely preserve the lamellar character of talc: the plastics containing as filler a talc thus calcined have a modulus of flexion much lower than that obtained with uncalcified talc or calcined talc according to the invention.

 The crushings, initial (a) and final (d) may consist of conventional mechanical grinding, associated with grain size selection operations, themselves conventional, which allow the elimination of unreduced particles (particle size greater than the diameter cutoff chosen). In particular it is possible to use either pendulum mills or hammer selector mills, or, preferably for the initial operation (a), a combination of the two types of mills (which makes it possible to achieve the lowest cost of the mills). indicated grain sizes).

 The agglomeration process (b) can take a known implementation in itself, consisting of a compression of the powder, in particular in the presence of a binder, at a pressure approximately between 10 and 100 bar; the binder may especially be water, provided in a weight proportion of approximately between 6 and 15. In particular, the agglomeration can be easily obtained at low cost by extrusion, by mixing the binder with the powder, by injecting it under pressure through dies and by cutting the cylinders obtained into sections.

 According to conditions of implementation giving good results, the grinding operations, initial (a) and final (d), are carried out so as to obtain a powder whose cutoff diameter D is between 5 and 50 microns, while the agglomeration step (b) is carried out in such a way as to obtain granules whose cutoff diameter D99 is between 4 and 15 mm, for example cylinder lengths of diameter between 3 and 8 mm and of length between 5 and 12 mm.

 In the process of the invention, the heating operation (c) of the granules is easy to perform and can be carried out by directly contacting the granules with combustion gases and stirring them in contact with these products. - a z. This operation can be carried out in a rotary kiln with direct flame, without generating the difficulties mentioned above n the experiments demonstrated that it is easy to control the residence time of the granules in the heating zone (zone of the oven in which the transformation temperature is reached), their flow being effected from regular fanon without tendan to the bonding at the expected temperatures and without inadvertent entrainment by the hot gases; the preferred temperature of the implementation is of the order of 1 1500 to 1 2200 C, and the residence time in the heating zone of the order of 5 to 30 n n. It was found that the product obtained no longer contained talc in its uncalcined state, and did not contain cristobalite (the accuracy of X-ray diffraction analysis).

 Preferably after the calcination (c), the products are cooled rapidly so as to bring their temperature to fall below 7000 C in less than 30 minutes, the final grinding operation (d) being carried out after this cooling. The latter can be executed from conventional fagot in a cold air cooler. Such cooling rules out any risk of failure of cristobalite even in trace amounts.

The method of the invention thus solves the problem of the industrial production of fine powder of calcined talc and combines the following results or advantages
obtaining a fine lamellar powder of enstatite and of amorphous silica, free of uncalcined talc and cristobalite,
easy industrial implementation by means of conventional type materials for each of the steps,
- loss of very limited raw materials leading to a very favorable yield,
- no pollution introduced during the processing stages and especially during grinding,
- energy consumption leading to costs compatible with the added value provided by the treatment.

 The calcined talc powder obtained can in particular be used as a filler in a polymerizable material (polyamides, polyolefins, polycarbomatas or others) to increase the rigidity without significantly reducing the impact resistance.

 The following examples are intended to illustrate the process of the invention (Examples 1 to 3) and the performance provided by products obtained in comparison with those of non-calcined products (Example 4).

EXAMPLE 1
a / This example was implemented from a talc ore "Luzenac 00" whose mineralogical composition is as follows (measurement thermopondérale analysis: "ATP"):
ra Talc 90 o
. Chlorite 8%
. Dolomite 1%
. Miscellaneous 1%
This ore first underwent two successive spraying operations
- grinding selection on pendulum mills "NEUMAM-ESSER PM 8", which gives a finished product whose granular characteristics are as follows (measurements "SEDIGRAPH 5000 D")
. D 99 = 40 microns
.D 50 = 10 microns
- grinding selection on "FORPLEX 52" hammer mill: the granulometric characteristics of the finished product which will have to be calcined in a complete way are the following ones
. D 99 = 15 microns
. D 50 = 3.5 microns
The selection rejects during this operation amount to about 50% by weight of the feed.

 During these two grindings, the mineralogy of the product has hardly changed.

 The method which is the subject of the invention was therefore tested under the most difficult conditions since the talc powder to be calcined is extremely fine (apparent density of 0.29 per volumeter "SCOTT NF T 30 - 042").

 This powder was then mixed with water (14% by weight) in a 2,000 liter mixer working continuously and equipped with a horizontal plow-share agitator.

The moistened powder is then introduced into a pellet press type "CALIFORNIA PELLET MILL
CENTURY 2 O CC "equipped with a die pierced with 5 mm diameter extrusion holes.

 At the outlet of the die, the rolls formed are cut by 2 sets of knives into sections of length equal to 8 minutes.

 The pressure applied by the rollers is 30 hars.

 The pellets are then conveyed by mechanical transport (screw and elevator) to the furnace feed hopper.

 During these operations, part of the granules are destroyed (about 10% by weight) but the "fines" fraction thus formed is retained to be introduced into the oven along with the mass of the granules.

c / The calcination was carried out in a conventional direct-fired rotary kiln
. Useful diameter = 0.5 m
. Length = 9 m
Burner food at "FOD"
The hot gas dewatering circuit includes
- 2 stages of cyclone,
- 1 fan
- 1 electrostatic precipitator.

The oven is equipped with three thermocouples (A,
B and C) connected to a recorder
Thermocouple A is placed near the introduction chute of the raw product.

 Thermocouple C is placed just upstream of the furnace heating zone.

 Thermocouple B occupies an intermediate position.

 The oven outlet temperature measurements are periodically made using a filament pyrometer.

 The feed rate of the furnace is regulated by means of a weighing-metering mat.

On leaving the oven, the calcined product falls into a rotary cooler:
. Length = 5 n
. Diameter = 0.6 n
The furnace setting parameters during the test are as follows:
. Pellet feed rate = 115 Kg / h (product
wet)
. Oven rotation speed = 2 rpm
. Combustible flow = 16 Kg / h
The combustion is done with a large excess of air (rate of oxygen leaving the oven = 10 g approximately) to limit the temperature of the flame to 1 2500 C.

After stabilization of the test (in charge of the furnace), the following measurements were carried out
. Finished product flow = 105 Kg / h
. Oven gas temperature = 6600 C
. Material temperature:
Thermocouple A = 3800 C
Thernocouple B = 7000 C
Thermocouple C = 9000 C
Oven output (end of the heating zone) = 1 2000 C
Cooling outlet = 350 C
. Residence time of the material in the oven = 75 mn
Time of passage in the heating zone = 20 mn
Time in the cooler = 25 minutes
At the outlet of the oven, the product is therefore instantaneously cooled below 8200 C, the limit temperature of transformation of talc into enstatite.

 The test was carried out on 3 tons of wet product, a little less than 30 hours of continuous walking.

 Excellent stability of the measured parameters was observed. The slope formed by the product in the oven is satisfactory and flows regularly. No clogging or accidental sticking was observed.

 Regular checks on the quality of the calcined product (one sample per hour) confirm the correct operation of the absence of talc and cristobalite lines on X-ray diffraction patterns.

A precise material balance was carried out on one ton of wet feedstock: recovered calcined product = 712 Kg
The loss on ignition at 1050 ° C. on the wet granulated product is 20% by weight corresponding to the removal of the granulation water (14% by weight) and the water forming the talc and chlorite ( 6% by weight) released during calcination. The loss of weight by removal of water during the calcination of one tonne of wet product is therefore 200 kg.

 The loss of material during the operation is therefore 88 kg. It can be considered that 50 of these losses take place in the furnace supply circuit and that the 50 o; The remaining ones are related to the dustiness in the oven.

 The losses of materials related to the calcination process are therefore kept to a reasonable level and will not penalize its industrial exploitation since the flown products recovered at the level of the filtration installations can be recycled upstream of the pellet mill.

d / The calcined product has a particle size distribution fairly close to the granulated starting product (slightly higher percentage of fines: linked to the mechanical stirring imposed by the oven)
The calcined product has an orange-pink color, which is oxidized by the traces of iron sulphides present in the initial ore.

 The consistency of the granules is much greater after calcination (very light sintering).

 The calcined products are then fed back to the selector mill "SiJPERBROYEUR FORPLEX S2" used to carry out the final phase of the grinding operation (a).

 The finished product from grinding is a fine powder having the same particle size distribution as the initial powder a).

 This operation is carried out without particular difficulties. The flow rate of the apparatus is twice as large as during operation a) and the refusals of selection are negligible (less than 1% by weight).

 This shows that the initial shape of the particles has been preserved, which is confirmed by the examination of the electron microscopic photographs, and that there is practically no sintering during the calcination.

 No abnormal wear of the mill is observed.

 Operation d) therefore has essentially a function of désaglonération.

 The powder thus obtained was analyzed by X-ray diffraction and by sedigraphy. It perfectly meets the requirements of the intended application.

EXAMPLE 2
a / This example was implemented by grinding the ore "Luzenac 00" under the same conditions as in Example 1.

 b / The manufacture of the granules was also carried out in the same manner as for Example 1.

 c) The calcination was carried out in the same rotary kiln and only the temperature was changed by varying the excess combustion air.

The oven settings were as follows:
. Pellet feed rate = 115 Kg / h
. Oven rotation speed = 2 rpm
. Combustible flow = 16 Kg / h
. Excess air such as the percentage of oxygen in the furnace exhaust gases, ie 12 t.

The measured parameters are as follows:
. Finished product flow = 105 Kg / h
. Oven gas temperature = 5600 C
. material temperature
Thernocouple A = 3700 C
Thermocouple B = 6800 C
Thermocouple C = 8600 C
Oven output = 1 0700 C
Cooling outlet = 360 C
. Time of stay in the oven = 75 minutes
. Residence time in the heating zone = 20 minutes
. Time in the cooler = 25 minutes
The flow of the granules in the oven is carried out correctly but there is a slight tendency to sticking upstream of the heating zone with formation of a ring on the refractory wall of the oven.

 The X-ray diffraction diagrams show that the calcination is not quite complete: a small talc peak remains.

 The test was therefore interrupted after two hours.

 d / The regrinding to 15 microns was carried out under the same conditions as for Example 1. The results are quite similar.

 Introduced into the polymer, the calcined talc of this example does not provide results as good as those obtained with the calcined talc of Example 1 (the temperature of the heating zone is below the optimum range of temperature).

EXAMPLE 3
This example was carried out by grinding the ore "Luzenac 00 under the same conditions as for Examples 1 and 2.

 b / The manufacture of the granules was also carried out in the same manner as for Examples 1 and 2.

 c / The calcination was carried out in the same rotary kiln and only the temperature was changed by varying the excess combustion air.

The oven settings were as follows:
. Pellet feed rate = 115 Kg / h
Oven rotation speed = 2 rpm
. Consistent flow = 16 Kg / h
Excbs of air such as the percentage of oxigen in the
gas conbustion out of oven, 8%.

The measured parameters are as follows
Finished product flow = 105 Kg / h
Oven gas temperature = 7800 C
. Material temperatures
Thermocouple A: 4000 C
Thermocouple B: 7500 C
Thernocouple C: 1 0000 C
Oven outlet temperature = 1 2400 C
. Cooling outlet temperature = 450 C
. Time of stay in the oven = 75 minutes
Residence time in the heating zone = 20 minutes
. Residence time in cooler = 25 minutes
The X-ray diffraction diagrams show a recrystallization of amorphous silica into cristobalite.

 The test was stopped after 1:35.

 d / The regrinding to 15 microns was carried out under the same conditions as for Examples 1 and 2.

 The presence of cristobalite does not modify the observed results (very low content).

 Introduced into the polymer, calcined talc in this example does not provide results as good as those obtained with the calcined talc of Example 1 (the temperature of the heating zone being above the optimum temperature range ).

EXEt1PLE 4
To determine what were the mechanical characteristics conferred by a non-calcined talc and a talc prepared according to the invention, in polypropylene, tests were carried out with the following talcs.
Talc I - non-calcined product - D99 = 15 microns
Talc II - product prepared in Example 1 - (D99 = 15 microns)
These talcs were treated conventionally in a fast mixer, with 1% of gamma aminopropyltriethoxysilane (coupling agent) and allowed to prepare the following mixture of products (weight composition):
60 S - Polypropylene homopolymer powder - milk index 3 "
38.6% - Talc
0.1% - Antioxidant :: Irganox 1010 (CIBA GEIGY) "
n, 3 S - Distearyl thiodipropiomate 1% - - Grafted polypropylene having an acid number of 47 and an approximate molecular weight of 4500
These powder mixes were used to feed a "Haake - Rheocord EU5 - Rheomix TW 100" laboratory. (temperature profile: 190 - 200 205 - 2150 C).

The ribbon blends are shredded and injection molded as tensile and flexural specimens. The mechanical characteristics are as follows

<tb>(<SEP>:<SEP>:<SEP>)
<tb>(<SEP>:<SEP> @split <SEP><SEP> elasticity <SEP> in <SEP>: <SEP> Resistance <SEP> at <SEP> shock <SEP>)
<tb>(<SEP>:<SEP> flex <SEP> (Mega <SEP> Pascals) <SEP>: <SEP>"Charpy"<SEP> no <SEP> notched <SEP>)
<tb>(<SEP>:<SEP>:<SEP> to <SEP> 23 <SEP> C <SEP>)
<tb> (Kilojoule / m2) <SEP>)
<tb>(<SEP>:<SEP>:<SEP>)
<tb>(<SEP>:<SEP>:<SEP>)
<tb>(<SEP> Talc <SEP> I <SEP>: <SEP> 3240 <SEP>: <SEP><SEP> 24.9 <SEP>)
<tb>(<SEP> Talc <SEP> II <SEP>: <SEP> 2580 <SEP>: <SEP> 49 <SEP>)
<tb>(<SEP>:<SEP>:<SEP>)
<Tb>
The polymer in the absence of filler has a modulus of elasticity of 1350 Megapascals and does not break in the impact test "Charpy" unscored at 230 C.

The addition of calcined talc according to the invention provides an excellent compromise modulus of elasticity / impact resistance.

Claims (10)

 1 / - Process for the preparation of fine powder of calcined calcined talc, characterized in that it consists in combining the following operations  (a) milling a talcous ore to reduce it to a powder having a cutoff diameter D99 of less than approximately 200 microns,  (b) agglomerating said powder to form granules, of particle size such that more than 70% by weight of the particles are larger than 500 microns,  (c) heating said granules at an elevated temperature of between 9000 C and 1 2500 C, so as to bring the granules to this temperature for the time necessary for their complete calcination,  (d) after cooling, grinding the calcined granules to obtain again a powder having a cutoff diameter D99 of less than approximately 200 microns.  2 / - Method according to claim 1, characterized in that the grindings (a) and (d) each consist of a mechanical grinding associated with a grain size selection operation, to remove particles of particle size greater than the cutoff diameter selected.  3 / - Method according to one of claims 1 or 2, characterized in that the agglomeration (b) consists of a compression of the powder, particularly in the presence of a binder, at a pressure of approximately between 10 and 100 bar .  4 / - Method according to claim 3, characterized in that in the agglomerating operation (b) is used a binder consisting of water in a proportion by weight approximately between 6 and 15 S.  5 / - Method according to one of claims 3 or 4, characterized in that the agglomeration operation (b) consists of mixing the binder with the powder, to inject it under pressure through dies, and to cut the cylinders obtained in sections.  6 / - Method according to one of claims 1, 2, 3, 4 or 5, characterized in that the heating operation (c) consists in directly disposing the granules in contact with gas combtlstion and brewing them in contact of these gases.  7 / - Method according to claim 6, characterized in that the heating operation (c) is carried out to bring the mass of talc granules at a temperature between 1 1500 C and 1 220 C for a period of between 5 and 30 minutes.  8 / - Method according to one of claims 1, 2, 3, 4, 5, 6 or 7, characterized in that, after the heating operation (c), the products are rapidly cooled. calcined to bring their temperature down to below 7000 C in less than 30 minutes.  9 / - Method according to one of the preceding claims, characterized in that  grinding operations (a) and (d) are performed so as to obtain a powder whose cutoff diameter D99 is between 5 and 40 microns,  . the agglomeration operation (b) is performed so as to obtain granules whose cutoff diameter D99 is between 4 and 15 mm.  10 / - Process according to claims 6 and 9 taken together, characterized in that the agglomeration operation (b) is performed so as to obtain cylinder sections of diameter between 3 and 8 mm and length between 5 and 12 mm.