CA1111216A

CA1111216A - Production of fibres with reduced shot formation

Info

Publication number: CA1111216A
Application number: CA283,789A
Authority: CA
Inventors: Alan T. Slater; Roy F. Preston; Ronald A.J. Sambell; David V.C. Jones
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1976-08-04
Filing date: 1977-07-29
Publication date: 1981-10-27
Also published as: BE857450A; JPS5319424A; LU77924A1; NL7708588A; FR2360521B1; DE2735236A1; AU2749677A; DK345577A; FR2360521A1; AU512628B2; ZA774542B; IT1085255B

Abstract

ABSTRACT FOR CANADA
Spray drying process and apparatus for producing fibres, especially polycrystalline inorganic fibres such as alumina, from viscous aqueous sols or solutions, optionally with the use of a linear organic polymer spinning aid such as poly-ethylene oxide. The preferred atomiser is a rotating disc atomiser which may have grooves in the disc vanes. The interior of the atomiser may be supplied with cold humid air to prevent premature solidification of the feed liquid which leads to shot formation. Hot air for drying the fibres is preferably introduced tangentially on to the interior wall of the drying chamber and away from the atomiser, and in the same direction as fibres are formed from the atomiser.
Fibres having mean diameters above 1 micron and less than 5 microns and virtually free from shot (non-fibrous material) can be produced.

Description

The present i.nvention relates to the production of fi.bres and finds an application ln the production of discontinuous refractory inorganic fibres.
Conventional spray-drying techniques may be used in the production of cliscontinuous inorganic fi.bres, but they tend to produce a high proportion of nonfibrous material commonly referred to as shot.
According to one aspect of the present invention there i.s provided . a process for the production of fibres by a spray-drying technique comprising feeding a fibre-forming material of a predetermined viscosity to an atomiser ~- in a spray-drying apparatus, air at substantially the same temperature as the fibre-forming material and having a relative humidity of not less than 50%
being introduced to the centre of the interior of the atomiser, the atomiser being set to favour fibre formation in preference to particle formation thereby to produce fibres of ~he fibre-forming material, maintaining the temperature in a drying chamber in the apparatus to give dried fibres and : -collecting the dried fibres under conditions such that the integrity of the fibres i.s not substantially affected.
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-In another aspec-t, the i.nvention provides a spray-drying apparatus f.or the production of fibres from a fibre-forming material comprising a drying-chamber, an atomiser in the drying chamber for forming a fibre-forming material i.nto fibres, means for introducing hot air into the drying-chamber tangentially onto a substantially cylindrical wall thereof to maintain the temperature in the dryi.ng chamber to gi.ve dried fibres, means for introducing air at substantially the same temperature as the fibre-forming material and of relat;.ve humidity not less than 50% to the centre of the i.nterior of the atomiser and means for collecting the dried f.ibres.
The fibre-forming material is preferably a fibre-forming liquid having a viscosity in the range of 1 to 200 poise preferably 20 to 200 poise.
:~ If necessary the fibre-forming liquid may include a spinning aid.
The fibre-forming material is preferably a fibre-forming liquid comprising one or more inorganic compounds which decompose on heating to form oxides. Preferred fibre-forming liquids.are aqueous.
Thus, khe fibre-forming material m~ be selected, for example, from certain inorganic sols (colloidal solutions3 which have a high degree of visco-elasticity as compared with the majority of inorganic sols. Examples of sols having a high degree of visco-elas*icity are denitrated zirconium nitrate and ~0 aluminium chlorohydrate. Such sols may be used with or without the addition of a spinning aid. ~ .

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Sols or solutions o~ inorganic compounds which do not have the rheological properties necessary for providing fibre-forming liquids themselves may be used with the addition of a spinning aid. Thus ~or example relatively dilute aqueous so~utions of metal salts of ~- organic acids such as zirconium and alumi~ium ` basic acetates advantageously comprise a :~
- splnning ald.
Suitable spinnin~ aids include organic polymers which are soluble in the fibre-forming liquid. Preferred organic polymers are lin~ar polymers, especially~polye~hylene oxide, partially hydrol~ysed polyvinyl acetate and polyvinylpyrrolidone. Polyethylene oxide having a low molecular weight~ for example:
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below 500,000:is espe~ially preferred as a ~. spinning aid. ~he spinning aid is used at -~ : a co~entration ln the fibre-forming liquid necessary to provlde the:required rheological : properties~ but it is pref rred for the con~entration to be below 10% by weight based :
on the solids content of the fibre-forming ~-. liquid expressed as oxidel for example from 2 to 6%.

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In one embodiment the atomiser in the spray-drying apparatus 7 which atomiser may be termed : "the fibrizer" in the context of the invention, can be a disc atomiser (also known in the art as a rotating vane atomiser) and is preferably controlled so as to have a disc peripheral speed in the range of lOm/s to 150m/s preferably 30 to 75m/s, depending upon the rheology (e.g. viscosity) of the fibre-forming material, so as to promote ~: 10 fibre ~ormation rather tha~ the production of ; particles~ :
-~ . In another embodiment a two-fluid:nozzle can be used as an atomi.ser (~ibrizer) ln the spray-drying apparatus:in which case the air ~:
pressure supplied to the two-fluid nozzle is . preferably in the range 50 to 150 p.s~i. to:
favour fibre formation.
: : In a further embodiment a rotating inverted bowl type of atomiser ~fibrizer) can be used.
In this case it is necessary to seek to ensure - ;~
that the fibre-forming material does;not slip over the sur~ace of the bowl. Peripheral i spe~ds are preferably controlled so as to be `
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for the disc atomiser.
The rate at which the fibre-forming liquid is fed to the atomiser (fibrizer) depends principally upon the dimensions of the atomiser. Thus in the case of a rotary vane atomiser, feed rate.s of ~etween abo.ut .~ 0.1 litres per minute and 3 litres per minute are typical for a disc of diameter of 120 mm.-~`. 10 Preferably to dry the ~ibres the temperature in the drying chamber of the spray-drying : . apparatus-is controlled to be in the range 50 to 100C when the fibre-forming liquid is a visco-elastic sol as hereinbefore~desc~ribed used without the addition of a spinning aid, since premature gelling:is liable to occur with higher drying temperatures. Fibre-forming liquids using a spinning aid are generally less concentra~ed and higher drying temperatures for - 20 example up to ~00C may be used with such liquids.
. In conventional spray-drying practice a steam of`hot air is used to provide heat for : drying and the arrangement of the apparatus is such that hot air enters the drying .~ chamber in a manner such that the atomiser '..'' , '.

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is immersed in a hot air flow. This can be used to give a turbulent air flow which assists particle formation.
From the point-of-view of producing fibres this is undesirable from two respects.
Firs~, this manner of hot air introduction can result in the atomiser (fibrizer) becoming undesirably hot. Secondly,it produces an air-flow pattern or turbulence which can seriously inhibit the formation of long fibres and fibres without malformations ., , :
(e~g~ kinks~.
Thus~ in accordance with the present ~` invention a preferred method of introducing ~!
hot air comprises introducing hot air tangentially into the drying chamber thereby ; to provide improved fibre-forming conditions.
The hot air is preferably introduced tangentially onto a sub~stantially cylindrical wall of the drying chamber.
Introduction of hot air in this way substantially avoids direct heating of the ;~
atomiser tfibrizer).

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. : , The direction of hot air introduction is ; preferably in the same direction as fibre ~ormation from the atomiser (fibrizer) in the case of a rotating disc or bowl atomiser.
The inlet temperature of the hot air is preferably maintained between 50C and 250C, the corresponding outlet temperatures being 40C to 150C.
However, the atomiser (fibrizer) will still reach substantially the same temperature as the drying chamber and, in the case where a rotatin~ vane atomiser (ibrizer) is used, hot ;~ air and debris (from the drying-chamber) will be pumped through the atomiser disc by an inherent centrifuyal pumping actionO
This can cause undesiralble premature ` solidification of the fibre-~orming material (e.g~ premature gelation in the case of a fibre-forming sol) within the atomiser ~fib~izer). This undesirable e$fect can be inhibited by introducing cold air (i.e.

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of the atomiser tfibri2er) disc.
The quantity of air 50 supplied to the interior of the disc preferahly should be at least slightly in excess of the air pumping capacity of the disc.
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In carrying out the process of the present invention it is also preferred that the mean relative humidity in the drying chamber is in the range 1 to 10%.
- 10Fibre collection can be batch-wise or continuous.
Batchwise collection is applicable to "pull-through" spray-drying where hot air is pulled through the spray-drying ~- 15apparatus by a fan operating ~n an exhaust - duct from the drying chamber. Spray-dr~ed product is earried out of the drying chamber in the air flow through the exhaust duct and i5 conveniently separated from this air flow by a cyclone separator.
Whilst this form of separation is acceptable for particulate spray-dry products, fibres tend to break up in the cyclone separator.

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Thus in accordance with the present invention it is preferred to use a wire mesh grid, in place of a cyclone separator, to collect fibres from the air flow without substantially affe~ting the integrity of the fibres (i.e.
without significantly damaging them).
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The wire mesh grid can be positioned so that hot air flows through it befor entering the exhaust duct and product is trapped on the grid.
In a typical spray-drier having a d~ying chamber with a cylindrical upper section and a . ,. ~
~` conical lower section the grid can ~e~positioned where the conical lower section begins.
:, - 15 Product has to be removed from the grid '~ frequently enough so that air flow dynamics in the drying chamber are not deleteriously affected.
~, Continuous fibre collection may be practiced .;
~-~ 20 in a "push-pull" or "two-fan" spray-dryer in which air is pushed into the drying chamber ~ , by one fan and pulled out therefrom by another fan. The two fans are adjusted to be balan~ed : ~ .
so as to allow fibre product to fall freely through a lower part of the drying chamber ~ which may conventionally be conical in section .', .

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and through a discharge orifice to a collector.
According to another aspect of the present invention a spray-drying apparatus for the production of fibres by forming fibres from a fibre-forming material comprises a drying-. .
chamber, an atomiser (fibrizer) in the drying~ , ~; chamber for forming a ibre-forming material into fibres, means for introducing hot air into the drying-chamber tangentiall;y onto a substantially ~- 10 cylindrical wall thereof and means for collecting dried fibres.
Preferably the atomiser (fibrizer) comprises a disc atomiser (i~e. rotating vane atomiser) especiall~ a dis~ atomiser having a plurality of fibre-forming edges at th~e~
` 15 periphery, said edges being profiled to provide ,.
~` a plurality of fibre-forming~sites.
The edges may be profiled to provide ~he plurality of fibre-forming sites by providing . ~
` a plurality of grooves in leading edges of each -~ 20 vare of the atomiser.

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It will be understood that the grooves perform a similar function to the plurality of holes in a conventional spinnerette in that many sites are provided for fibre-formation. Preferably a means is providedfor the tangential intro~uction of hot air away from the vicinity of the atomiser (fibrizer) disc by directing the hot air onto the wall of the drying chamber. Also it is preferred that the means for the tangential introduction is such that air can be introduced into the ~; drying chamber in the same direction as :
fibre-formation from the a~Gmise~ (fibrizer) disc when in operation.
The spray~drying apparatus according to the present invention may be provided with means for introducing humid air to the centre of the interior of the atomiser (fibrizer) disc.
The means for collecting dried fibres in accordance with one embodiment of the invention ; may be a wire mesh grid arranged in the path , of fibre-containing exhaust air leaving the drying chamber such that fibre is 'filtered' from the exhaust alr by retention on the grid.

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The means Eor collecting dried fibres in accordance with another embodiment of the invention may be an orifice in a lower part of the drying chamber and a collector (e.g.
a conveyor-belt or a container) positioned ...
below the orifice.
; The invention will now be further described, by way of example only, with reference to the ~ingle figure of the accompanying drawing which shows a portion of a rotating vane atomiser ~- ~fibrizer) in accordan~e wlth the present invention~ The drawing is not to scaleO
Referring now to the dra~ing there is ~`
shown an edge portion of a rotating vane (dlsc) atomiser (fibriæer) 1 for posltioning in a drying chamber of spray-drying apparatus, said atomiser 1 having a plurality of radial vanes ~ (two only shown) terminating at the i periph~ry 3 of the a~omiser 1.
~ 20 The leading edges of the vanes 2 are ,, providing with a plurality of grooves 4 ~
which give a plurality of troughs at the ~ -edges formed by the peripheral terminations of the vanes 2, thereby to form on each edge a piurality of sites for fibre-formation " in operationO
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It will be appreciated that ~or convenience of illustration only two of the vanes 2 are shown and that similar vanes 2 are provided around the entire atomiser (~ibrizer) 1~
In operation the atomiser tfibrizer) 1 : is positioned in a drying-chamber of a spray-drying apparatus and rotated in the : direction indicated by the arrow 5.
Fibre-forming liquid is fed to the ato~iser (fibrizer) 1 and mov~s outwar~ly under centrifugal force in the approximate direction indicated by arrows 6 along the radical vanes 2, emerging at the troughs provided by the grooves 4 at the peripheral terminations of the vanes 2.
Th~ fibre-forming ma~erial is flung off the rotating atomiser (fibrizer) 1 and drawn înto a plurality of fibres.
The fibres are then dried in the drying :
chamber to give dried fibres.
It will be appreciated that were it not for the grooves 4 fibres would possibly be drawn from one or more random positions on each leading edge rather than from a plurality of predetermined positions.
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British Patent No 1,265,394 (UKAEA) relates ~o the production of inorganic fibres from sols by spray-drying to give gel fibres, follwed by calcination to give calcined, inorganic fibres and as hereinbefore mentioned in accordance with , ...
~ an embodiment of the present invention similar .
fibre-forming sols can be used as the fibre-forming material ln which case the f~bres collected will be gel fibl-es which can be ~`~ 10 calcined to give a polycrystalline fibre ~, product. Fibres from other fibre-forming .` materials comprising inor~ganic compounds ,~' which decompose on heating to form oxides may also be calcined to form polycrystalline : :
s 15 oxide fibres, fvr example at temperatures j~ in the range 500C to 1500C. -Therefore by starting~with the appropriate ~;~
fibre-forming sol or solution polycrystalline ; ; fibres can:~e made whieh comprise for example - 20 alumina, zlrconia and silica and mlxtures ;~ thereofO Magnesia,~calcia or yttria can:be introduced into the f.ibres for example by incorporation of the respective nitrate or oxide in the fibre-forming sol or solution.
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According to a further aspect the presen~ invention provides a fibre produced by the process of the invention.
Conventional spray drying especially without a spinning aid does not reproducibly yield satis~actory fibres since the product tends to be sub-micron in diameterr lmm in length and a potential health hazard.
It i~ believed that the present invention can be used to substantially ` avoid or overcome the foregoing dis-advantages.
The invention will now be illustrated by the following ~ ~ -A denitrated zirconium ~itrate/calcium nitrate sol was prepared in accordance with the procedures described in British Patent No.
l,181,794 so as to have a viscosity of 100 poise and an e~uivalent oxide content of 0~740 kg~litra (the composition was such that the zirconia product would contain 6 wt% calcia).
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This sol was supplied at a rate of 0.1 litres per minute to a rotating vane disc atomiser (Eibrizer) in a pull-throuyh spray -~ dryer and fibres were produced. The peripheral speed of the disc atomiser was 50m/s and the temperature in the drying chamber of the spray dryer was 80C. The sol temperature was 30C at the point of distribution onto the ~' atomiser ~fibrizer) disc. Air at 30 and 85% Relative Humidity was supplied to the interior of the disc at a rate o~ 20m3 per hour.
"~, The fibres were drled in the spray drylng apparatus and the product which contained 95%
fibre was collected on a wire mesh grid.
~; 15 Subsequently the fibres were~ calcined to give . .
poly~rystalline fibres typically o~ 5 microns diameter and 2 cm length~

An aqueous solution was prepared having the i 20 following composition.
Aluminium oxychloride solution in water (23.5~ by weight as A1203) 75.5~ by weight Aluminium oxychloride powder (47~ by weight~as A1203) 18.9% by weight Polysiloxane polyether copolymer (20~ by weight as SiO2) 5.7% by weight Polyethylene oxide (mean molecular weight 0.79% by weight 300,000) , . : .

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. , This composition had a viscosity of 20 poise at 25C and had an equivalent oxide content such that the fibre product would contain about 95~ A1203 amd 5% SiO2 by weight.
The composition was fed at a rate of 150 ml per minute to a rotating vane disc atomiser ~- .. (fibrizer) in a ~push-pull' (two fan) spray dryer and fibres were produced. The disc diameter was 120mm.and it was rotated at a peripheral speed . 10 of 57m/s. The temperature of the feed composition was 35C at the point o~ di~tribution onto the atomi~er di~c~ Air at 35C and at a Relative Humid:ity of 96% was su~plied to the interior of the ~;; dis~ at a rate of 50m3 per hourO The temperature : 15 of the drying air was 180C at the inlet and 60C
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- The dried fibres were collected on a wire mesh grid and calcined. The polycrystalline fibres of alumina/ ~.,!

~; silica obtained had a medlan diameter of 2.5 mic~ons;

~ 20 0~1% by weight had a diameter smaller than 1 micron; :~

- 3~ by weight had a diameter greater than 5.5 micronsl 95~ by:wei~ht had a diameter smaller than 4.5 microns.

Only 1-2% by weight of non-fibrous material (shot) . .
~ was present in the fibre product.

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An aqueous solution was prepared having the following compositionO
Aluminium oxychloride ~ solution in water ~`i'7"'7 23.5~ by weight as A120 3) 73.9% by weight Aluminium oxychloride powder ~47% by weight at Al603) 18.8% by weight ~ -Polysiloxane polyether copolymer (20% by weight as SiO2) 5.68% by weight Polyethylene oxide (mean molecular weight lo ~ 200,000~ 5-7~ by IA/E~
This composition was adjusted to a viscosity of 10 poise at ~5C by the addition of wa~er. It had an ~` equivalent oxide content such that the fibre product would contain about 55% Al203 and 5% SiO2 by weight.
The composition was fed at a rate of 50ml per minute to a rotating van disc atomiser ~fibrizer) in a "push-pull"~
(two-fan~ spray dryer and fibres were produced. The dlsc ~ diameter was ~20mm and it was rotated at a peripheral -;' : :
speed of 94m/s. The temperature of the feed composition 20 was 35C at the point of distribution onto the atomiser disc. Air at 35C and at a Relative Humidity of 95% was supplied to the interior of the disc at a rate of 70m3 per hour. The temperature of the drying alr was l20C

~' at the inlet and 40C at the outlet.
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The dried fibres were collected on a wire : mesh grid and calcined. The polycrystalline fibres of alumina/silica obtained had a median diameter of 1.5 microns; 40% by weight had a diameter smaller than 1 micron; 2% by weight had a diameter greater than 2.5 microns; ~5%
by weight had a diameter smaller than 2 microns. :
Only 1-2% by weight of non-fibrou~ material (shot) was present in the fibre product.

_, An a~ueous solution was prepared having the composition shown in Example 2.
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~. . The composition was fed.at a rate o.f ~00 ml - per minute to a~rotatlng van~disc atomiser (flbrizer) in a "pull-push" ~two fan~ spray dryer and fibre were:produced. The disc diameter i;
was 120mm and it was rotated at a peripheral speed o~ 38 m/s. The temperature of the feed composition was 35C at~the point of distribution onto the ' atomiser disc. Air at 35C and at a Relative ~ Humldity of greater than 95% was supp1ied to the ; : interior of the disc at a rate of 50m3 per hour.
The temperature of the:drying air was~220C at the inlet and 8~C at the~outlet.
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The dried fibres were collected on a wire mesh grid and calcined. The polycrystalline fibres of alumina/silica obtained had a median diameter of 4.5 microns; 0.1~ by weight had a diameter smaller than 2.5 microns; 95~ by weight had a diameter smaller than 6.5 microns. There was 3~ ~y weight of non-fibrous material (shot) ~ in the .ibre product.

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S UF'PLEMEN'~RY DISC ll~S URE

In the main disclosure there are described a process and an appara-tus for the production of fibres by a spray-drying technique wherein during the production of fibres air at substantially the same temperature as -the fibre-forming material and of relative humidity not less than 50% is introduced to -the centre of the atomiser of a spray-drying apparatus.
; We have found, however, that providing the air introduced to the atomiser is moist, i.e, has a relative humidity greater than 50%, then the temperature of -the air need not be substantially the same , as the temperature of the fibre-forming material. In particular we have found that the temperature of the moist air introduced to the atomiser may be less than or greater than the temperature of the fibre-forming material, for example up to 60C in the case where the fibre forming naterial is at or about ambient temperature (say 30C
to 35C). Preferably the temperature of the air is from 40C to 60C irrespective of the temperature of the fibre-forming material.
Especially preferred is the introduction to the atomiser of moist ~;~ air àt a temperature of from 45C to 55C. Preferably the air has a relative humidity of at least 80%.
We have found also that whilst introducing the moist air to the centre of the interior the atomiser is convenient in practice, it is sufficient for the production of satisfactory fibres that the moist air be introduced to the spray-drying apparatus at any point where it becomes entrained in the interior of the atomiser. Thus for example the moist air may be introduced at the periphery o~ the atomiser or at a point exter~lal to the atomiser from ~hich it is drawn into the atomiser by the pumping action of the atomiser in operation. Usually, since the temperature in the apparatus external to the atomiser will often be higher than 60C, it is more convenient to introduce the moist air directly into the atomiser. The type of atomiser employed in the s~dryiny apparatus is immaterial to the benefit obtained by introducing moist air to the atomiser.
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Thus, in accordance with the present ;nvention, there is provided a process for the production of fibres by a spray-drying technique comprising feeding a fibre-forming material of a predetermined vi.scosity to an atomiser in a spray-dryi.ng apparatus, air havi.ng a relative humidity of not less than 50% bei.ng :introduced to the inter1.or of the atomiser, the atomiser being set to favour fibre formation i.n preference to parti.cle formation thereby to produce fibres of the fibre-forming material maintaini.ng the temperature in a drying chamber in the apparatus to give dried fibres and collecting the dried fibres ~mder conditions such that the integrity of the fibres is not substantially af:Eected.
The invention also provides a spray-drying apparatus for the production of fibres from a fibre-forming material comprising a drying chamber, an atomi.ser in the drying-chamber for forming a fibre-forming material into fibres, means for introducing hot air into the drying-chamber tangentially onto a substantially cylindrical wall thereof to maintain the temperature in the drying-chamber to give dried fibres, means for introducing air of relative humidity not less than ., 50% to the interior of the atomiser, and means for collecting the dried fibres.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the production of fibres by a spray-drying technique comprising feeding a fibre-forming material of a predetermined viscosity to an atomiser in a spray-drying apparatus, air at substantially the same temperature as the fibre-forming material and having a relative humidity of not less than 50% being introduced to the centre of the interior of the atomiser, the atomiser being set to favour fibre formation in preference to particle formation thereby to produce fibres of the fibre-forming material, maintaining the temperature in a drying chamber in the apparatus to give dried fibres and collecting the dried fibres under conditions such that the integrity of the fibres is not substantially affected.

2. A process as claimed in claim 1, in which the fibre-forming material is a fibre-forming liquid having a viscosity in the range 1 to 200 poise.

3. A process as claimed in claim 1 or 2 in which the fibre-forming material is a fibre-forming liquid comprising one or more inorganic compounds which decompose on heating to form oxides.

4. A process as claimed in claim 1 in which the fibre-forming material is selected from inorganic sols which have a high degree of visco-elasticity.

5. A process as claimed in claim 4 in which the inorganic sol is denitrated zirconium nitrate or aluminium chlorohydrate.

6. A process as claimed in claim 1 in which the fibre-forming material is a fibre-forming liquid which includes a spinning aid.

7. A process as claimed in claim 6 in which the spinning aid is a linear organic polymer.

8. A process as claimed in claim 7 in which the linear organic polymer is polyethylene oxide, partially hydrolysed polyvinyl acetate or polyvinylpyrrolidone.

9. A process as claimed in claim 8 in which the polyethylene oxide has a molecular weight below 500,000.

10. A process as claimed in claim 6, 7 or 8 in which the spinning aid is used at a concentration in the fibre-forming liquid below 10% by weight based on the solids content of the fibre-forming liquid expressed as oxide.

11. A process as claimed in claim 1 in which the fibre-forming material is an aqueous fibre-forming liquid.

12. A process as claimed in claim 1 in which the atomiser is a disc atomiser.

13. A process as claimed in claim 12 in which the peripheral speed of the disc of the atomiser is in the range 10 m/s to 150 m/s.

14. A process as claimed in claim 13 in which the peripheral speed of the disc of the atomiser is in the range 30 m/s to 75 m/s.

15. A process as claimed in claim 1 in which the temperature in the drying chamber of the spray-drying apparatus is up to 300°C.

16. A process as claimed in claim 15 in which the temperature in the drying chamber of the spray-drying apparatus is controlled to be in the range 50 to 100°C.

17. A process as claimed in claim 16 in which hot air is introduced tangentially into the drying chamber.

18. A process as claimed in claim 17 in which the hot air is introduced tangentially onto a substantially cylindrical wall of the drying chamber.

19. A process as claimed in claim 17 or 18 in which the atomiser is a rotating disc or bowl atomiser and the hot air is introduced in the same direction as fibre formation from the atomiser.

20. A process as claimed in claim 1 in which the air has a relative humidity of greater than 95%.

21. A process as claimed in claim 1 in which the quantity of air introduced to the centre of the interior of the atomiser disc is at least slightly in excess of the air pumping capacity of the disc.

22. A process as claimed in claim 1 in which air in the drying chamber has a mean relative humidity in the range 1 to 10%.

23. A process as claimed in claim 1 in which the fibres are collected on a wire mesh grid.

24. A spray-drying apparatus for the production of fibres from a fibre-forming material comprising a drying-chamber, an atomiser in the drying chamber for forming a fibre-forming material into fibres, means for introducing hot air into the drying-chamber tangentially onto a substantially cylindrical wall thereof to maintain the temperature in the drying chamber to give dried fibres, means for introducing air at substantially the same temperature as the fibre-forming material and of relative humidity not less than 50% to the centre of the interior of the atomiser and means for collecting the dried fibres.

25. A spray-drying apparatus as claimed in claim 24 in which the atomiser is a disc atomiser.

26. A spray-drying apparatus as claimed in claim 25 in which the disc atomiser is a rotary vane atomiser having a plurality of fibre-forming edges at its periphery, said edges being profiled to provide a plurality of fibre-forming sites.

27. A spray drying apparatus as claimed in claim 26 in which the fibre-forming edges at the periphery of the disc are profiled to provide a plurality of grooves in leading edges of each vane of the atomiser.

28. A spray-drying apparatus as claimed in claim 25 in which the means for the tangential introduction of hot air is arranged so that the hot air is introduced into the dry-chamber in the same direction as fibres are formed from the atomiser disc.

29. A spray-drying apparatus as claimed in claim 25 in which means is provided for introducing humid air to the centre of the interior of the atomiser disc.

30. A spray-drying apparatus as claimed in claim 24 in which the means for collecting dried fibres is a wire mesh grid arranged in a path of fibre-containing exhaust gas leaving the drying-chamber.

Claims based on Supplementary Disclosure

31. A process for the production of fibres by a spray-drying technique comprising feeding a fibre-forming material of a predetermined viscosity to an atomiser in a spray-drying apparatus, air having a relative humidity of not less than 50%
being introduced to the interior of the atomiser, the atomiser being set to favour fibre formation in preference to particle formation thereby to produce fibres of the fibre-forming material, maintaining the temperature in a drying chamber in the apparatus to give dried fibres and collecting the dried fibres under conditions such that the integrity of the fibres is not substantially affected.

32. A process according to claim 31 wherein said air having a relative humidity of not less than 50% has a temperature of from about 40°C to 60°C.

33. A process according to claim 32 wherein the temperature of the moist air is from about 45°C to 55°C.

34. A process according to claim 31 or 32 wherein the relative humidity of the moist air is at least 80%.

35. A spray-drying apparatus for the production of fibres from a fibre-forming material comprising a drying chamber, an atomiser in the drying-chamber for forming a fibre-forming material into fibres, means for introducing hot air into the drying-chamber tangentially onto a substantially cylindrical wall thereof to maintain the temperature in the drying-chamber to give dried fibres, means for introducing air of relative humidity not less than 50% to the interior of the atomiser, and means for collecting the dried fibres.