CA1073184A - Production of improved chrysotile asbestos fibres - Google Patents

Abstract

PRODUCTION OF IMPROVED CHRYSOTILE ASBESTOS FIBRES
ABSTRACT OF THE DISCLOSURE
The filtration rate of asbestos-cement slurries formed from chrysotile fibres is improved by controlled heat treatment of the fibres without loss of fibre strength.
The improved filtration rate allows an increased rate of production of asbestos-cement products.

Description

73i84 This invention relates-to a process for producing chrysotile asbestos fibres with improved properties when used as a reinforcing agent for asbestos-cement products.

Chrysotile asbestos is a fibrous or asbestiform magnesium silicate mineral and is the only member of the serpentine sub-group of asbestos. The most important members of the other sub-group of asbestos, that is, the amphiboles, are amosite and crocidolite. The individual fibrils of chrysotile, measuring about 300 to 400 A in diameter, have a scroll-like structure consisting of alternating layers of silica and brucite. The hydroxyl groups of the outermost brucite layer represent the surface of the fibrils. In contrast, the silica layer represents the surface in amphi-boles and this fundamental difference is the reason for most of the differences in the physical properties of the two materials.
After conventional mining and milling, chrysotile asbestos is available as ~ibres, up to about 5 mm in diameter, which are in the form of bundles of fibrils~ These bundles may be sub-divided further by mechanical action, and the degree of sub-division of the fibres is generally expressed in terms of average surface area of the product. Chrysotile asbestos fibre produced by a conventional mill ranges in surface area between about 3030 and 5000 cm /g and is generally provided in pressure packed fo~m with a density of about 25 to 50 lb/ft3.
Chrysotile asbestos is used in the manufacture of asbestos-cement products such as pipes and sheets. The manufacturing process consists of filtering a sl~ry of asbestos and portland cement (or portland cement partially

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substituted with silica flour) through a filter drum rotating in a vat. The thin layer of asbestos-cement formed continuously on the drum is transferred first to an endless filtration felt that moves over suction boxes, and is subse-quently wound onto a mandrel.
It is necessar~ for this procedure for the chrysotile asbestos to be opened prior to ormation of the slurry thereof with the cementl since the fibres produced by the asbestos mill are too coarse ~or effective use in this -procedure. Opening of chrysotile fibres consists of mechanical action to separate these fibres into more fibres of smaller diameter. During the opening of the fibres, shorten-ing of the fibres to a limited extent is unavoidable. It is known that fibres of shorter length provide lesser strength characteristics than fibres of longer length.
Water is removed from the asbestos-cement slurry at two stages of the above procedure: (1) on the filter drum, and (2) at the suction boxes. The rate with which the water can be removed is one of the most important properties of the a~bestos-cement slurry, because the rate of water removal determines the rate of production of the products. In general, when chrysotile asbestos is used as the sole asbestos fibre in the slurry, the filtration rates obtained are poor.
It is common practice therefore to substitute up to 30% by weight of the chrysotile by other naturally occurring asbestos fibres, namely, amosite (white asbestos) and crocidolite (blue asbestos), which possess high filtration rates, in order to improve the overall filtration properties of the asbestos cement. However, amosite contributes little to the flexural strength of asbestos-cement products while crocidolite has been associated with health problems and its use in ~73189L

industrial applications is either banned or subject to severe restrictions. Moreover, both materials are in short supply.
Attempts to improve the filtrat:ion properties of asbestos-cement slurries by the addition of surfactants, coagulants, etc. have met with limited success. Similarly, treatment of the chrysotile asbestos with sodium silicate has been only partially successful in improving the filtration properties. Heat treatment of chrysotile asbestos has been proposed as a means of improving the filtration properties of chrysotile fibres but considerable strength reductions of the asbestos-cement products have resulted when this method was used and hence, has generally been considered unsatisfactory.

It has now been found that ~ailure of the prior heat treatment suggestion to achieve higher filtration rates without substantial strength loss is due to lack of control in the heat treatment. It has been found that both the temperature and the time of the treatment must be maintained within certain narrow limits to obtain th~ desired improvement in filtration properties without loss in strength. Treatments at higher temperatures even for shorter times, will result in o~ertreatment of asbestos fibres with small diameters, producing a corresponding strength loss. Conversely, fibres with larger diameters will be undertreated on exposure to lower tempera~ures even when longer times are used.
In accordance with the present invention~ chrysot~le asbestos fibres are heat treated under controlled conditions at a temperature between about 400C and about 600~C, while controlling the heating of the fibres to avoid a significant degree of dehydroxylation of the asbestos fibres. It is the . ~

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dehydroxylation of the asbestos fibres which leads to the undesirable strength losses mentioned above.
Chrysotile asbestos fibres which are heat treated in accordance with this invention exhibit increased filtra-tion rates in asbestos-cement uses without significant strength losses. The heating is effected for a time sufficient to increase the filtration rate to at least that of crocidolite fibres of approximately the same surface area.
In a preferred embodiment of the invention, the chrysotile fibres are heated at a temperature of about 500 to 550C for 2 to 3 hours while the fibres are in a pressure packed form having a density of about 25 to about 50 lbtft3.
The heating process according to the invention changes the physical characteristics of the asbestos fibres.
Most importantly, the fibres become stiffer and more coherent ,`
because of an increase in the interfibrillar bond. Thus, if the fibres are subject to mechanical action a~ter the heating step, there results a greater degree of shortening of the fibres during their separation than would be the case if the mechanical action were carried out prior to the heating step.
Shortening of the fibres leads to strength lossesl as mentioned previously.
To minimize strength losses due to fibr~ shor~ening in the preferred procedure of the present invention, the chrysotile ~ibres are opened, typically to an average surface area in the range of about 8000 to 12,000 cm /g prior to pressure packing of the opened fibres for heating. The chrysotile fibres then are heat treated, for example at ;
about 500C for 2 hours.

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When this pre~erred procedure is carried out, the asbestos fibre formed exhibits iltration properties in asbestos-cement use comparable to those exhibited by the ,.

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amphiboles, crocidolite and amosite, without significant s~r ngth losses~
When the opening and heating steps are reversed, the fibres produced also exhibit an improvement in filtration properties, but,as noted above, this procedure is less desirable because of the resulting reduction in strength.
If the raw fibre to be heat ~reated according to this invention has a low average surface area, it is desirable first to classify the fibres tG remove a coarse fraction therefrom, which ~ay represent approximately 10 to 25% by weight of the original fibre sample, the fine ~raction then being subjected to the procedure according to the invention.

The invention is illustrated by the following Examples:
Example I
Chrysotile asbestos fibres (Group 5), opened to a surface area of 10,000 cm2/g, were pressure packed to a density of 50 lb/ft3 and heated at 500C for 2 hours.
Following heat treatment and cooling, the fibres were dispersed by light mechanical action. The procedure was repeated except that t~e raw fibres were irst pressure packed and heat treated and then opened following the heat treatment.
The properties of the materials with respect to asbestos-cement production were determined and compared with those of untreated fibres, amosite and crocidolite. The results are reproduced in the following Table I:

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73~4 The results of the above Table I quite clearly demonstrate the improvement in filtration rate achieved by the heat treatment procedure and that strength loss occurs only if the fibre is heat treated before openingO
Example II
Chrysotile asbestos fibres (Group 5), opened to a surface area of 11,000 cm /g, were pressure packed to a density of 50 lb/ft3 and heated at 550C for 3 hours.
Following heat treatment and cooling, the fibres were dispersed by light mechanical action. .
The properties of the treated fibres when used to prepare asbestos-cement products were detexmined and compared with those of untreated fibres, crocidolite, and subsitutions of up to 30% of the lntreated fibre by treated fibre or crocidolite~ The results are reproduced in the following Table II: ~ .

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The superiority of the heat treated fibres in asbestos-cement use is again demonstrated by the results of the above Table II.
Example III
Chrysotile asbestos fibres (Group 4) were formed into a pressure-packed block and heated at 500C for 2 hours in a muffle furnace~ Following the heat treatment and cooling of the block, the fibres were opened to an a~verage surface area o about 7000 cm2/g using a fan opener.
The opened fibres were fed to a Centri-Sonic classifier operating under conditions which will produce 10 to 15% rejects. (heavy fraction~
The fines fraction from the classifier was tested for its properties in asbestos-cement. These properties were compared with those of untreated fibres which had been otherwise processed to produce their optimum properties.
The results are reproduced in the following Table III~

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7331~1~4 The results of the above Tahle III demonstrate that the heat treated, opened and fine classified fibres produced in accordance with the invention exhibit substantially no decrease in modulus of rupture and an increase in asbestos- ~
cement filtration rate of about 75%. `
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The present invention, therefore, provides a pxo-cedure for producing treated chrysotile asbestos fibres which have improved filtration properties without exhibiting substantial strength losses. Modifications are possible within the scope of the invention.

SUPPLEMENTARY DISCLOSURE
In the principal disclosure, there is described a procedure for improving the filtration properties of chrysotile asbestos fibres in asbestos cement uses without any overall significant strength loss. The procedure involves heating the chrysokile asbestos fibres at a temperature of about 400 to about 600C while controlling the heating to avoid any significant degree of dehydroxylation of the fibres. The heating is achieved by placing the fibres in an ambient atmosphere having a substantially uniform temperature, thereby permitting the fibres to attain the ambient temperature and avoid overheating during the heat treatment.
In accordance with this supplementary disclosure, the heat treatment is effected for a period of about 1 to about 5 hours while controlling the temperature-time relation-ship of the heating to avoid loss of water of crystallization.
The maximum water loss (i.e., dehydroxylation) which can be ` ' tolerated is about 2~ by weight of khe total water of crystallization, and preferably without any dehydroxylation.
2Q As indicated in the principal disclosure, the heat treatment isi~Ee~e~b~-effected to increase the filtrakion rate from the value for untreated fibres in asbestos-cement uses ti.e., about ll to about 12 ml/sec) to a value compar~ble to that of crocidolite in asbestos-cement uses (i.e., about l9-to about 20 ml/sec), although higher values may be attained.
In this way, it is possible to provide asbestos fibres having a high filtration rate in asbestos-cement uses without the arawbacks of the amphiboles.
While the process of the invention has wide applica-bility to a variety of grades of chrysotile asbestos fibres, it has parkicular utility with respect to those chrysotile asbestos .. ..

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fibres classified as Group 4 and Group 5 by Quebec Asbestos Mining Association, since these classes of fibres are among those most commonly used in asbestos cement.
As set forth in the principal disclosure, it is preferred to carry out the heat treatment on the fibres in pressure packed form, since pressure packing decreases consid~
erably the volume of the fibres to be treated and permits -~
commercial operation, since large volumes of fibres can be heat treated simultaneously.
When in the pressure packed form, i~ takes some time for the heat to permeate the block, so that fibres in outer portions of the block may be heat treated at the ambient atmosphere temperature for longer periods than those towards the centre of the block. This heating pattern results in an ;
overall improvement in filtration properties in asbestos-cement uses for the fibres in the block without strength loss, although the individual filtration properties of the fibres in the block may vary. The pressure pack typically is formed from a 100 lb lot of fibres and may be dimen~ioned 24" x 16" x 9".
The following example illustrates the invention further.
Example IV
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Chrysotile asbestos fibres (Group 5), opened to a - surface area of 10,000 sw.cm./g, were pressure packed into samples ¦ having a density of 50 lb/cu.ft. The opened fibres had an initial - asbestos cement filtration rate of 11 ml/sec, a modulus of rupture value of 424 kg/sq.cm and a water of crystallization content of 12% by weight, as determined by weight loss on ignition.

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Each sample was heated in an ambient atmosphere for a `
particular time period at a temperature in a muffle furnace. In each case, the filtration and strength properties of the heat treated fibres were ~etermined along with the residual water of :
crystallization of the fibres, and these values are tabulated as the following Table IV: ~:
TABLE IV :
Treatment Parameter Temperature (C) Time (hrs) 300 400 500 600 1 FR(l) 11 11 12 14 LOI(2) 12 12 12 11.9 MOR( ) 425 420 429 416 LOI 12 12 12 11.8 .~ . , 3 FR 11 13.5 19 24 : : :
LOI 12 12 12 10.9 MOR 439 422 414 360 ~;
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4 FR 11. 17 21 28 LOI 12 12 11.7 10 ~ ~ .

LOI 12 12 11.4 9.1 MOR 410 450 389 300 `:
Notes: (1) FR i5 the asbestos-cement filtration rate of . treated fibre in ml/sec. .:
(2) LOI is the weight loss of the heat treated fibres in % and is the measure of the water of crystalliza-tion content of the fibres.
(3) MOR i~ the modulus of rupture of asbestos cement in kg/cm . ~hese values are accurate .only within +5~.

It will be seen from the results of the above Table 73~8~
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IV that a minimum temperature of 400C is required to achieve any improvement in asbestos cement filtration properties and the heating time must be controlled within the temperature range of 400 to 600C, if no more than 2~ by weight of the total loss of water of crystallization is to be achieved, corresponding to 0.25% by weight loss in the LOI values. The l: -modulus of rupture data in Table IV indicates a trend of loss of strength with loss of water of crystallization, as is evident from the following Table V:
TABLE V

LOIWater loss by heat MOR value Temp/time (wt.%)treatment heating % Of fibre wt. ~ total H2O ]cg/sq.cmO ~C/hrs) 11.9 0.1 0.8 416 600/1 11.8 0.2 1.7 402 600/2 :~ -11.7 0.3 2O5 391 500/4 11.4 0.6 5.0 389 500/5 10.9 1.1 7.6 360 600/3 As can be seen from Table IV , the asbestos-cement filtration rate for the fibres generally increases with the length of time of heating but a point is reached in each case when further heating, while further increasing the asbestos ; . ~Ent filtration rate, leads to loss of water of crystallization and strength loss. ~;

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for the production of chrysotile asbestos fibres having improved filtration properties in asbestos-cement uses, which comprises heating chrysotile asbestos fibres at a temperature of about 400°C to about 600°C for a time sufficient to increase the filtration rate of said fibres in asbestos-cement uses to at least: the filtration rate of crocidolite fibres of approximately the same average surface area, and controlling said heating to avoid any significant degree of dehydroxylation of said fibres during said heating, whereby said improved filtration properties are obtained without any significant overall fibre strength loss.

2, The method of claim 1 wherein said heating is carried out at a temperature of about 500 to about 550°C for 2 to 3 hours while the fibres are in a pressure-packed form having a density of about 25 to about 50 lb/ft3.

3. The method of claim 1 including opening said chrysotile fibres from an average surface area of about 3000 to about 5000 cm2/g to an average surface area of about 8000 to about 12,000 cm2/g prior to said heating step.

4. The method of claim 3 including pressure packing said opened fibres after said opening step and prior to said heating step to a density of about 25 to about 50 lb/ft3.

5. The method of claim 1, 2 or 3 including classifying said chrysotile asbestos fibres into a coarse fraction and a fine fraction prior to said heating step and subjecting said fine fraction to said heating step.

6. The method of claim 1, 2 or 4 wherein said heating is carried out in a muffle furnace.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

7. A method for the production of chrysotile asbestos fibres having improved filtration properties in asbestos-cement uses, which comprises heating chrysotile asbestos fibres selected from the Group 4 and Group 5 chrysotile asbestos fibres having a filtration rate in asbestos cement uses of about 11 to about 12 ml/sec in an ambient atmosphere having a substantially uniform temperature of about 400° to about 600°C for a time from about 1 to about 5 hours sufficient to increase the filtration rate of said fibres in asbestos-cement uses to at least about 19 ml/sec and insufficient to result in a loss of water of crystallization of the fibres of greater than about 2% by weight of the total weight of water of crystallization in the fibres, whereby said improved filtration properties are obtained without any significant overall fibre strength loss.

8. The method of claim 7 wherein said heating is effected to effect substantially the maximum increase in said filtration properties without any loss of water of crystallization from the fibres,