Method for manufacturing of mineral fibres

Abstract

The present invention relates to a method for manufacturing mineral fibres comprising 35–55 weight-% of SiO2, 10–30 weight-% of Al2O3 and 1–15 weight- % of Fe2O3. The method comprises introduction of a raw material mixture into a furnace, melting it to form a mineral melt, which is formed into mineral fibres. Impregnated glass fibre material is introduced into the furnace, as a part of the raw material mixture. The glass fibre material is impregnated with an inorganic additive and comprises at least 30 weight-% of Al2O3 and / or at least 35 weight-% of CaO, calculated from inorganic content of the impregnated glass fibre material.

Description

[0001] METHOD FOR MANUFACTURING OF MINERAL FIBRES

[0002] The present invention relates to a method for manufacturing of mineral fibres according to the preamble of the enclosed independent claim.

[0003] Glass fibre non-wovens are produced from chopped glass fibres by bonding or interlocking the glass fibres by mechanical means, such as fibre entanglement, and / or chemical means, such as binder. The glass fibres as such are heat resistant and relatively fire resistant, as they are non-combustible. However, glass fibres melt around 550 - 600 °C. The binders used for production of glass fibre non-wovens, e.g. for interlocking the fibres into a fixed structure, may have a lower melting point or they may even inflame at high temperatures. In order to improve the heat and / or fire resistance of the glass fibre non-wovens, the non-woven can be impregnated with an inorganic additive, such as calcium carbonate or aluminium trihydrate. These impregnated glass fibre non-wovens are used, for example, in construction industry.

[0004] During the production of impregnated glass fibre non-wovens as well as similar impregnated fibre materials, a certain amount of waste is always generated. For example, the edges of the impregnated glass fibre non-woven must be trimmed, creating trimmings that are discarded. Sometimes the final product does not meet the stringent quality requirements and must be discarded as off-specification production. There is no effective way of recirculating the trimmings and / or off-specification production within the production process itself, as the impregnation with the inorganic additive makes the chemical composition of the glass fibre nonwoven unsuitable for remelting within the production process of glass fibres.

[0005] There are increasing obligations for producers to take responsibility for the entire lifecycle of their products, in particular at the end of the product’s life. In some countries, these obligations already require packaging industry and producers of some consumer goods, such as electronic appliances, to take back used and discarded products. It is anticipated that these obligations will spread also to other industry sectors. This means that the producers of impregnated glass fibre non-wovens and similar impregnated fibre materials might be required to take back their products, e.g. from construction and / or demolition sites.

[0006] There is clearly a need to find a way to exploit discarded and / or recycled impregnated glass fibre material. It’s disposal to a landfill is not a sustainable or an acceptable option, at least in a long run.

[0007] It is an object of the present invention to reduce or even eliminate the above-mentioned problems appearing in prior art.

[0008] One object of the present invention is to provide a method with enables use a of novel raw material in manufacture of mineral fibres, especially for insulation purposes.

[0009] A further object of the present invention is to improve the resource management and sustainability in the manufacture of mineral fibres.

[0010] In order to realise the above-mentioned objects, among others, the invention is characterised by what is presented in the characterising part of the enclosed independent claim.

[0011] Some preferred embodiments according to the invention are disclosed in the dependent claims presented further below.

[0012] The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, even if this is not always separately mentioned.

[0013] Atypical method according to the present invention for manufacturing mineral fibres comprising 35 - 55 weight-% of S iC>2, 10 - 30 weight-% of AI2O3 and 1 - 15 weight-%, preferably 2.5 - 15 weight-%, of Fe2Os, comprises

[0014] - introducing a raw material mixture into a furnace,

[0015] - melting the raw material mixture in the furnace to form a mineral melt, -forming the mineral melt into mineral fibres, and- introducing into the furnace, as a part of the raw material mixture, an impregnated glass fibre material impregnated with an inorganic additive and comprising at least 30 weight-% of AI2O3 and / or at least 35 weight-% of CaO, calculated from inorganic content of the impregnated glass fibre material.

[0016] Now it has been surprisingly found that the impregnated glass fibre materials, which have been impregnated with an inorganic additive, comprise a composition of inorganic oxides which makes them especially suitable for manufacture of mineral fibres comprising 35 - 55 weight-% of SiC>2, 10 - 30 weight-% of AI2O3 and 1 - 15 weight-%, preferably 2.5 - 15 weight-%, of Fe2Os, i.e. for manufacture of mineral fibres known as “stone wool” fibres. The high content of AI2O3 and / or CaO in the impregnated glass fibre material, which have earlier prevented its recycling within the production of glass fibres, makes it uniquely suitable for incorporation into the raw material mixture for the manufacture of “stone wool” fibres, and solves many problems within their manufacture. For example, aluminium oxide is an important component in these mineral fibres, and expensive raw materials are often needed to provide the desired level of aluminium oxide to the mineral melt and to the produced mineral fibres. Now it was found that the impregnated glass fibre material can provide the desired aluminium content, at least partly, thus reducing the raw material costs and the need for virgin raw materials.

[0017] In the present context the term “impregnated glass fibre material” denotes a glass fibre material, preferably glass fibre non-woven, which has been impregnated after the formation of the glass fibre material with an inorganic additive. The inorganic additive does thus not form a part of the composition of the glass fibre itself, but the inorganic additive has been incorporated into the structure of the material produced from glass fibres, such as non-woven, by any conventional impregnation technique, such as immersion, spraying or the like. The production of impregnated glass fibre materials, such as glass fibre non-wovens, is known as such for a person skilled in the art and does not form a part of the present invention. Typically the impregnated glass fibre material comprises glass fibres, a binder and the inorganic additive. The binder is usually an organic binder, which binds the glass fibres into a coherent material, e.g. non-woven.The inorganic additive impregnated to the glass fibre material is preferably calcium carbonate or aluminium trihydrate. The inorganic additive is typically used to increase the heat resistance of the impregnated glass fibre material, such as nonwoven, or to provide it with other desired features. For example, aluminium trihydrate may be used to provide fire and heat resistance.

[0018] According to one preferable embodiment, the impregnated glass fibre material is a glass fibre non-woven, impregnated with the inorganic additive. Examples of impregnated glass fibre non-wovens comprise veils for ceiling boards, gypsum boards, flooring boards and wall boards. Impregnated glass fibre non-woven may be, for example, impregnated chopped strand mat. The glass fibre non-woven usually has a structure of randomly oriented glass fibres, forming to a coherent structure with a binder. The amount of binder in the glass fibre non-woven may be 1 - 30 weight-%, often 5-25 weight-%, calculated from the total weight of the glass fibre non-woven. The glass fibres may have a fibre length of 0.1 - 200 mm. The glass fibre non-woven may be formed by any suitable technique, e.g. by wet-laying. The impregnated glass fibre non-woven may have a weight in a range of 10 - 300 g / m2, typically 15 - 200 g / m2or 20 - 150 g / m2

[0019] The impregnated glass fibre material may be a woven glass fibre material, such as fabric, roving or the like, impregnated with an inorganic additive.

[0020] The impregnated glass fibre material used in the present invention may be a waste material, such as edge trimmings, and / or off-specification products originating from production of impregnated glass fibre non-wovens, such as mats, veils, sheets, blankets or fabrics, or it may be a recycled impregnated glass fibre non-woven, or any mixture thereof. The waste material or off-specification products originating from the production of impregnated glass fibre non-woven may comprise only one specific non-woven product quality or it may be a mixture of several different nonwoven product qualities, e.g. with different amounts of inorganic additive and / or with different inorganic additives in each product quality. In the present context, the recycled impregnated glass fibre non-woven denotes impregnated glass fibre non-woven that has left its original production site and that has been returned to the original non-woven producer, their representative or to the distributor, either as used or unused. The unused recycled glass fibre non-woven may be, for example, impregnated glass fibre non-woven returned to the producer or distributor, e.g. due to damaged packaging, or as waste or surplus from a production of secondary products, such as ceiling, floor and / or wall boards, such as trimmings, wrongly cut pieces or the like. Some unused recycled glass fibre non-woven may originate from construction sites. The used recycled impregnated glass fibre non-woven may be sourced from demolition sites or from used ceiling, floor or wall boards.

[0021] The impregnated glass fibre material forms a part of the raw material mixture, fed into a furnace, where the raw material mixture is melted and a mineral melt is formed. Any conventional melting method can be used. The furnace may be an electric furnace, electric arc furnace, submerged combustion furnace, cyclone furnace, gas- or coke-fired furnace, or a shaft furnace, such as cupola furnace.

[0022] According to one preferable embodiment, the raw material mixture comprising the impregnated glass fibre material is fed and melted in an electric furnace. The electric furnace may use either graphite electrodes or molybdenum electrodes for forming the mineral melt from the raw material mixture. It was surprisingly noticed that when the raw material mixture for the mineral melt comprises the impregnated glass fibre material, especially impregnated glass fibre material impregnated with aluminium trihydrate, the occurrence of flame formation on top of the melt bath is significantly reduced or fully eliminated. The raw material mixture for mineral fibre manufacture usually comprise variable amounts of fibrous material with organic binder, from the mineral fibre production process itself, such as spinning chamber waste, trimmings and the like. It is known that the organic binder may ignite when the fibrous material is melted, which may lead to flame formation on top of the melt bath in the electric furnace. Flame formation is naturally undesired as it increases the fire risk in the production process. The impregnated glass fibre material comprises usually organic binder and it was expected that it would increase the occurrence of the flame formation. However, when the raw material mixture comprise impregnated glass fibre material the flame formation in the electric furnace is effectively suppressed. Itis assumed, without wishing to be bound by any theory, that the inorganic additive impregnated into the glass fibre material produces the effect observed.

[0023] The impregnated glass fibre material may be shredded before it is introduced into the furnace, as a part of the raw material mixture. Any suitable shredding apparatus can be used. The shredding, in the present context, encompasses also cutting or clipping of the glass fibre material into smaller pieces. The shredded impregnated glass fibre material may be introduced into the furnace as such, or it may be formed into pellets or briquettes before its introduction into the furnace, as described below.

[0024] The impregnated glass fibre material may be formed into a compacted body, such as pellet or briquette, before its introduction into the furnace as a part of the raw material mixture. The impregnated glass fibre material may be shredded, and optionally mixed with a binder, before it is formed into a compacted body.

[0025] According to one embodiment, the impregnated glass fibre material may be formed into pellets before it is introduced into the furnace as a part of the raw material mixture. In the present context, the term “pellet” denotes a rounded, spherical or cylindrical body comprising at least the impregnated glass fibre material. The impregnated glass fibre material, such as impregnated glass fibre non-woven, may be shredded, compacted and formed into pellets. The pellets comprise or consists of impregnated glass fibre material and a binder, such as thermoplastic binder. The binder may be, for example, an epoxy resin, a polyester resin or a polyvinyl alcohol resin. The binder may originate from the impregnated glass fibre material itself or additional binder can be mixed with the (shredded) impregnated glass fibre material before formation of the pellets. Optionally, the pellets may even comprise one or more additional inorganic materials, such as a filler. The use of filler may be beneficial for adjusting the chemical composition of the pellets or their physical properties, e.g. strength. The pellets may comprise 50 - 100 weight-%, preferably 70 - 98 weight-%, more preferably 80 - 95 weight-%, of impregnated glass fibre material. The pellets may typically have a maximum dimension in a range of 1 - 50 mm, preferably 2 - 40 mm, more preferably 5 - 30 mm. The pellets may have a density within the range of 150 - 1000 kg / m3, preferably 300 - 800 kg / m3, providingflowability and easy handling due to reduced fluffiness. When the impregnated glass fibre material is formed into pellets, the dusting during handling of the impregnated material can be minimised.

[0026] Shredded or pelletized impregnated glass fibre material is especially suitable for melting in electric furnaces.

[0027] According to another embodiment the impregnated glass fibre material may be formed into briquettes before it is introduced into the furnace. The briquettes comprise at least impregnated glass fibre material, preferably as shredded, and a briquette binder. According to one embodiment, the briquettes may comprise up to 50 weight-%, preferably up to 40 weight-%, more preferably up to 30 weight-%, of impregnated glass fibre material, calculated from the total weight of the briquette. For example, the briquettes may comprise 1 - 50 weight-%, preferably 2 - 40 weight-%, more preferably 4-30 weight-% or 5 - 20 weight-% of impregnated glass fibre material, calculated from the total weight of the briquette. The briquette binder may be a hydraulic binder, such as Portland cement, lime, blast furnace slag or cement kiln dust, clay or any mixture thereof. The briquette binder may be a polysaccharide, polysaccharide derivative, such as sugar- or starch-based binder, or any mixture thereof. For example, the briquette binder may be a polysaccharide selected from starch, modified starch and molasses, preferably molasses. Preferably the briquette binder is Portland cement. Furthermore, the briquettes may comprise an additional inorganic mineral material, such as quartz sand, apatite, dolomite, olivine sand, limestone, kaolin, ilmenite, bauxite, rutile, magnesite, magnetite, slags from iron and / or steel manufacture, such as blast furnace slags, electric arc furnace slags, or any mixtures thereof. According to one preferable embodiment, the briquettes comprise, in addition to the impregnated glass fibre material, also recycled waste obtained from the production process of mineral fibres, such as spinning chamber waste, trimmings and mineral fibre material discarded during production process.

[0028] Briquettes comprising impregnated glass fibre material are especially suitable for melting in a shaft furnace, such as cupola furnace.The impregnated glass fibre material may comprise relatively low amounts of SiO2, Fe20s and / or alkali metal oxides Na2O+K2O. This is advantageous, as it enables use of relatively large amounts of impregnated glass fibre material in the raw material mixture, without causing deviations from the desired composition for the mineral fibres. The impregnated glass fibre material may comprise 5-35 weight-%, preferably 7 - 30 weight-%, of SiC ; and / or 0.01 - 0.5 weight-%, preferably 0.01 -0.2 weight-%, of Fe2Os, and / or <1 weight-%, preferably 0.01 - 0.5 weight-%, of Na2O+K2O, calculated from the inorganic oxide content of the impregnated glass fibre material.

[0029] According to one preferable embodiment the impregnated glass fibre material may comprise 30 - 80 weight-%, preferably 50 - 77 weight-%, more preferably 60 - 75 weight-%, of AI2O3, calculated from the inorganic oxide content of the impregnated glass fibre material. This is especially applicable, when the impregnated glass fibre material comprises aluminium trihydrate as the inorganic additive.

[0030] When the impregnated glass fibre material comprises aluminium trihydrate as the inorganic additive, the impregnated glass fibre material may comprise, in addition to described percentages of AI2O3, 10 - 35 weigh-%, preferably 12 - 30 weight-%, more preferably 15 - 25 weight-%, of SiC ; and / or 1 - 15 weigh-%, preferably 3 -12 weight-%, more preferably 5 - 10 weight-%, of CaO, calculated from the inorganic oxide content of the impregnated glass fibre material. The Fe2Os content is preferably <1 weight-%, preferably <0.5 weight-%, calculated from the inorganic oxide content of the impregnated glass fibre material. The high aluminium content makes this impregnated glass fibre material especially suitable as raw material for manufacture of bio-soluble mineral fibres. The high aluminium content makes it also possible to reduce the use of expensive high aluminium raw materials, such as bauxite. At the same time, the amount of CaO and SiO2 are on a relatively low level, ensuring that their amount in the raw material mixture remains at the desired level.

[0031] The impregnated glass fibre material may comprise 35 - 80 weight-%, preferably 37 - 75 weight-%, more preferably 50 - 70 weight-% or 40 - 60 weight-%, of CaO.calculated from the inorganic oxide content of the impregnated glass fibre material. These values are especially applicable, when the impregnated glass fibre material comprises calcium carbonate as the inorganic additive.

[0032] When the impregnated glass fibre material comprises calcium carbonate as the inorganic additive, the impregnated glass fibre material may comprise, in addition to described percentages of CaO, 3 -40 weight-%, preferably 5 - 35 weight-%, more preferably 7 - 30 weight-%, of SiC ; and / or 0.5 - 12 weight-%, preferably 1 - 10 weight-%, more preferably 1.5 -8 weight-%, of AI2O3, calculated from the inorganic oxide content of the impregnated glass fibre material. The Fe2Os content is preferably <1 weight-%, preferably <0.5 weight-%, calculated from the inorganic oxide content of the impregnated glass fibre material.

[0033] According to one embodiment, the impregnated glass fibre material may comprise >10 weight-%, preferably 10 - 30 weight-%, more preferably 12 - 25 weight-%, of TiO2, calculated from the inorganic oxide content of the impregnated glass fibre material.

[0034] All compositions given in the present context comprise, in addition the specific oxides described, smaller amount of inorganic oxides conventionally present in the mineral fibres, such as MgO. The total amount of these inorganic oxides may be 0.5 - 5 weigh-%, preferably 1 - 4 weight-%, more preferably 1.2 - 3 weight-%, calculated from the inorganic oxide content of the impregnated glass fibre material.

[0035] According to one preferable embodiment, the impregnated glass fibre material may be sorted into two or more fractions on basis of AI2O3 content and / or CaO content, before the impregnated glass fibre material is incorporated into the raw material mixture, optionally in form of compacted bodies as described above, and introduced into the furnace. The impregnated glass fibre material is thus introduced into the furnace as sorted fractions. The sorting of the impregnated glass fibre material is preferably done before formation of the compacted bodies, such as pellets or briquettes, more preferably before shredding. The AI2O3 content and / or CaO content of the impregnated glass fibre material may be determined by any suitable analyticalmethod, e.g. by using X-ray fluorescence (XRF) analysis. The determination may be automated.

[0036] According to one embodiment, the raw material mixture introduced into the furnace may comprise impregnated glass fibre material in amount of >0.5 weight-%, preferably >0.75 weight-%, more preferably >1 weight-%, calculated from the total weight of the raw material mixture. For example, the raw material mixture introduced into the furnace may comprise impregnated glass fibre material in amount of 0.5 -35 weight-%, preferably 0.75 - 25 weight-%, more preferably 1 - 15 weight-%, even more preferably 1 - 10 weight-% or 1 - 6 weight-%, calculated from the total weight of the raw material mixture. The present invention thus enables the use of high amounts of recycled or waste material as a raw material for mineral fibres, especially for insulation purposes. In addition to the impregnated glass fibre material, the raw material mixture comprises conventional raw materials, i.e. crushed mineral material, such as basalt and / or diabase.

[0037] The formed mineral melt can be formed into mineral fibres by using any conventional fiberising method. The mineral fibres can be formed by using internal centrifuging, such as spinning cup, or external centrifuging, such as cascade spinner. The formed fibres are collected in form of a fibre web, which is formed and / or cut into desired products.

[0038] The formed mineral melt and consequently the formed mineral fibres comprise 35 -55 weight-% of SiC>2, 10 - 30 weight-% of AI2O3 and 1 - 15 weight-% of Fe2Os, i.e. they have chemical composition which defines them as “stone wool” fibres, not as glass fibres. According to a preferable embodiment the formed melt and the formed mineral fibres may comprise 37 - 50 weight-%, preferably 38 - 48 weight-%, of SiC ; 12 - 27 weight-%, preferably 13 - 25 weight-%, of AI2O3; and 2.5 - 15 weight-%, preferably 3 - 14 weight-%, preferably 3.5 - 12 weight-%, of Fe2Os. In some advantageous embodiments, the formed mineral melt and the formed mineral fibres may comprise 1.5 - 15 weight-%, preferably 2 - 14 weight-%, preferably 2.5 - 12 weight-%, of Fe2Os. The iron is given, as conventional in the art, as Fe2Os, but it may be present in the melt as FeO, or as a mixture of Fe2Os and FeO. The highcontent of iron oxide makes the formed fibres to show good resistance to fire. The formed mineral melt and consequently the formed mineral fibres may further comprise 3 - 35 weight-%, preferably 5 - 30 weight-%, more preferably 7 - 25 weight-%, of CaO; 0- 15 weight-% or 0.01 - 15 weight-%, preferably 1 - 13 weight-%, more preferably 1 - 12 weight-%, of MgO; and 0.5 - 17 weight-%, preferably 1 - 15 weight-%, more preferably 1.5 - 12 weight-%, of Na2O + K2O. The formed mineral melt and the formed mineral fibres may further comprise fibres comprise 0.01 - 3 weight-%, preferably 0.05 - 2.5 weight-%, of TiC ; and optionally further 0.01 - 3 weight-% of MnO; 0.01 - 3 weight-% of P2O5; and / or 0.01 - 3 weight-% of B2O3. The different ranges described above can be freely combined with one another. The sum of the oxide percentages is 100%.

[0039] The formed mineral melt and consequently the formed mineral fibres comprise 35 -55 weight-% of SiC>2, 10 - 30 weight-% of AI2O3 and 1 - 15, preferably 2.5 - 14 weight-% of Fe2Os, i.e. they have chemical composition which defines them as “stone wool” fibres, not as glass fibres. According to one preferable embodiment the formed melt and the formed mineral fibres may comprise 39 - 53 weight-%, preferably 40 - 52 weight-%, of SiC ; 12 - 27 weight-%, preferably 13 - 25 weight-%, of AI2O3; and 3 - 14 weight-%, preferably 3.5 - 12 weight-%, of Fe2Os. The iron is given, as conventional in the art, as Fe2Os, but it may be present in the melt as FeO, or as a mixture of Fe2Os and FeO. The high content of iron oxide makes the formed fibres to show good resistance to fire. The formed mineral melt and consequently the formed mineral fibres may further comprise 3 - 35 weight-%, preferably 5-30 weight-%, more preferably 7-25 weight-%, of CaO; 0- 15 weight-%, preferably 0.5 - 12 weight-%, more preferably 1 - 12 weight-% or 1 - 10 weight-%, of MgO; and 0.5 - 17 weight-%, preferably 1 - 15 weight-%, more preferably 1.5 - 12 weight-%, of Na2O + K2O. The formed mineral melt and the formed mineral fibres may further comprise small amounts of additional inorganic oxides conventionally present in mineral melts for mineral fibres, such as P2O5, B2O3, or TiO2, as described above. The different ranges described above can be freely combined with one another. The sum of the oxide percentages is 100%.According to the invention impregnated glass fibre material, impregnated with an inorganic additive, is used as part of a raw material mixture for manufacture of mineral fibres comprising 35 - 55 weight-% of SiC>2, 10- 30 weight-% of AI2O3 and 1 - 15 weight-%, preferably 3 - 14 weight-%, of Fe2Os, wherein the impregnated glass fibre material comprises at least 30 weight-% of AI2O3 and / or at least 35 weight-% of CaO, calculated from inorganic oxide content of the impregnated glass fibre material.

[0040] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.

Claims

CLAIMS1. Method for manufacturing mineral fibres comprising 35 - 55 weight-% of SiC>2, 10 - 30 weight-% of AI2O3 and 1 - 15 weight-% of Fe2O3, the method comprising - introducing a raw material mixture into a furnace,- melting the raw material mixture in the furnace to form a mineral melt, -forming the mineral melt into mineral fibres,characterised inintroducing into the furnace, as a part of the raw material mixture, an impregnated glass fibre material, impregnated with an inorganic additive and comprising at least 30 weight-% of AI2O3 and / or at least 35 weight-% of CaO, calculated from inorganic content of the impregnated glass fibre material.

2. Method according to claim 1, characterised in that the impregnated glass fibre material is a glass fibre non-woven, impregnated with the inorganic additive.

3. Method according to claim 2, characterised in that the impregnated glass fibre material is a waste material and / or off-specification products originating from production of impregnated glass fibre non-wovens, a recycled impregnated glass fibre non-woven, or any mixture thereof.

4. Method according to claim 1 , 2 or 3, characterised in that the impregnated glass fibre material is shredded before it is introduced into the furnace as a part of the raw material mixture.

5. Method according to any of preceding claims 1 - 4, characterised in that the impregnated glass fibre material is formed into pellets before it is introduced into the furnace as a part of the raw material mixture.

6. Method according to any of preceding claims 1 - 4, characterised in that the impregnated glass fibre material is formed into briquettes before it is introduced into the furnace.

7. Method according to claim 6, characterised in that the briquettes comprise up to 50 weight-%, preferably up to 40 weight-%, more preferably up to 30 weight-%, of impregnated glass fibre material, calculated from the total weight of the briquette.

8. Method according to any of preceding claims 1 - 7, characterised in that the impregnated glass fibre material comprises 30 - 80 weight-%, preferably 50 - 77 weight-%, more preferably 60 - 75 weight-%, of AI2O3, calculated from the inorganic oxide content of the impregnated glass fibre material.

9. Method according to any of preceding claims 1 - 8, characterised in that the impregnated glass fibre material comprises 35 - 80 weight-%, preferably 37 - 75 weight-%, more preferably 50 - 70 weight-%, of CaO, calculated from the inorganic oxide content of the impregnated glass fibre material.

10. Method according to any of preceding claims 1 - 9, characterised in that the impregnated glass fibre material comprises- 5 - 35 weight-%, preferably 7 - 30 weight-%, of SiC ; and / or- 0.01 - 0.5 weight-%, preferably 0.01 - 0.2 weight-%, of Fe2Os, and / or- <1 weight-%, preferably 0.01 - 0.5 weight-%, of Na2O+K2O,calculated from the inorganic oxide content of the impregnated glass fibre material.

11. Method according to any of preceding claims 1 - 10, characterised in that the impregnated glass fibre material comprises >10 weight-%, preferably 10 - 30 weight-%, more preferably 12 - 25 weight-%, of TiO2, calculated from the inorganic oxide content of the impregnated glass fibre material.

12. Method according to any of preceding claims 1 - 11, characterised in that the formed mineral fibres comprise 37 - 50 weight-%, preferably 38 - 48 weight-%, of SiO2; 12 -27 weight-%, preferably 13 - 25 weight-%, ofA^Os; and 2.5- 15weight-%, preferably 3 - 14 weight-%, more preferably 3.5 - 12 weight-%, of Fe2O3.

13. Method according to any of preceding claims 1 - 12, characterised in that the formed mineral fibres comprise 3-35 weight-%, preferably 7-25 weight-% of CaO.

14. Method according to any of preceding claims 1 - 13, characterised in that the formed mineral fibres comprise 0- 15 weight-%, preferably 1 -13 weight-% of MgO; and 0.5 - 17 weight-%, preferably 1 - 15 weight-% of Na2O + K2O.

15. Method according to any of preceding claims 1 - 14, characterised in that the formed mineral fibres comprise 0.01 - 3 weight-%, preferably 0.05 - 2.5 weight-%, of TiO2.

16. Method according to any of preceding claims 1 - 15, characterised in that the raw material mixture introduced into the furnace comprises impregnated glass fibre material in amount of 0.5 - 35 weight-%, preferably 0.75 - 25 weight-%, more preferably 1 - 15 weight-%, calculated from the total weight of the raw material mixture.

17. Method according to any of preceding claims 1 - 16, characterised in that the impregnated glass fibre material is sorted into two or more fractions on basis of AI2O3 and / or CaO content, before introducing the glass fibre material into the furnace.