BRIQUETTES FOR MINERAL FIBRE PRODUCTION AND THEIR USE

Abstract: A process is provided for the production of man-made vitreous fibres having a composition which contains at least 14 % aluminium measured as weight Al2O3 based on oxides, comprising providing in a shaft furnace a self-supporting stack of a mineral charge which includes briquettes, melting the charge to provide a melt at the base of the furnace having the composition of the fibres, removing melt from the base of the furnace and fiberising the melt, and at least one quarter of the aluminium in the charge is introduced as particulate alumina-containing mineral in briquettes and the particulate alumina-containing mineral contains from 0.5 to 10 wt.% metallic aluminium, 50 to 90 wt.% alumina Al2O3 and 0 to 49.5 wt.% other materials.

Description: Briquettes for Mineral Fibre Production and Their Use This invention relates to processes of making high alumina content man-made vitreous fibres (MMVF) from a mineral charge which includes briquettes, and to briquettes for this purpose. MMVF may be made by forming a mineral melt by melting a mineral charge in a furnace and fiberising the melt, usually by a centrifugal fiberising process. In many of the furnaces which are used, there is a large pool of melt and the mineral charge is melted into this pool. Examples are tank and electric furnaces. In such furnaces, the physical form (i.e., lump or powder) of the mineral charge is relatively unimportant since the melting is conducted into a large volume of previously melted material. However, there is another type of furnace which is used for forming the melt for MMVF production, especially of fibres of the types that are referred to as rock (including stone or slag) fibres. This is a shaft furnace in which the furnace contains a self-supporting column of solid coarse mineral material and combustion gases permeate through this column so as to heat it and cause melting. The melt drains to the bottom of the column, where a pool of melt is usually formed, and the melt is removed from the base of the furnace. Since the column has to be both self- supporting and permeable it is necessary that the mineral material should be relatively coarse and should have considerable strength, despite the high temperatures in the column (which may exceed 1000°C) . The mineral material can be formed of coarsely crushed rock and slag provided this will withstand the pressures and temperatures in the self-supporting column in shaft furnace. It is known to convert the finer particulate materials such as sands into bonded briquettes for addition to the furnace. These should have sufficient strength and temperature resistance to withstand the conditions in the self-supporting column in the shaft furnace in order that they melt prior to collapsing. It is necessary for the total charge in the furnace (i.e., crushed mineral alone or crushed mineral plus briquettes) to provide the composition which is desired for the MMV fibres which are to be made. There is a particular interest in the manufacture of MMVF insulation containing more than 14%, and often 18 to 30%, alumina, for instance as described in 096/14274 and W096/14454. These mention the general concept of use of waste materials as part of the starting material. These include high alumina (20 to 30%) slags such as ladle slag, filter dust and high alumina waste from the production of refractory materials. W096/14274 describes the production of specified physiologically soluble fibres in various ways, including methods using various furnaces such as electric furnaces and cupola furnaces. The use of aluminium-containing waste materials in general is indeed now known, and in electric furnaces and other furnaces discussed above in which the charged mineral materials are melted directly into a pool of melt the waste material can generally be charged directly into the melt pool in any form, usually as received. O97/30002 specifically describes the use of bauxite. In practice bauxite (calcined and/or uncalcined) is the material which has been most widely proposed and used for the manufacture of such fibres. Unfortunately bauxite is a relatively expensive raw material and the use of bauxite incurs difficulties (additional to its cost) in a shaft furnace containing a self-supporting stack of mineral. In a shaft furnace bauxite must be charged in a form which can form part of the self-supporting stack. Thus it can be charged as coarse rock. In shaft furnaces the residence time of material in the small melt pool at the base of the furnace is short, and the raw materials must melt sufficiently rapidly in this pool of melt if a melt is to be obtained which is suitable for provision of final product having good properties . Bauxite requires a high energy input for melting, in particular if provided in the form of coarse rock. Bauxite can also be provided as part of the briquette component, which requires major energy expenditure to crush and grind the bauxite into a suitable form. However, even when ground into fine particles and incorporated into briquettes, bauxite gives melting problems due to its high melting point. In fact, a proportion of the bauxite does not melt at all but is instead dissolved in the pool of melt at the base of the f rnace. To maximise melting of the bauxite in the time available requires provision of fuel, in particular solid fossil fuel such as coke. This increases costs and improves melting, but even then inevitably a small proportion of the bauxite is not melted completely. The unmelted bauxite accumulates in the bottom of the furnace. This means that the melt exiting the furnace does not have exactly the same composition as the charge of raw mineral materials. Furthermore, the accumulated bauxite reduces the volume of the melt pool and the residence time in that pool is thus reduced further. Consequently the accumulated unmelted bauxite must be removed from the furnace from time to time. In the production of high alumina fibres using a shaft furnace, it is frequently necessary to provide a large proportion of the charge as briquettes. It would be desirable to be able to provide briquettes which have good resistance to the high temperatures and pressures in the shaft furnace and thus can form a strong self-supporting stack but also melt sufficiently rapidly and uniformly that they release their constituents evenly into the melt. In particular it would be desirable to provide briquettes which have improved properties in comparison with briquettes containing ground bauxite . Thus the invention is concerned with the specific problems which arise with the use of bauxite and most alumina-containing waste materials in briquettes in shaft furnaces. We have found that processes for the provision of high alumina fibres from briquettes, preferably physiologically soluble fibres, in a shaft furnace can be improved by the selection of specific raw materials having defined content of alumina and metallic aluminium. According to the invention we provide a process of producing man-made vitreous fibres having a composition which contains at least 14% aluminium (measured as weight Al 2 0 3 based on oxides) by providing in a shaft furnace a self-supporting stack of a mineral charge which includes briquettes, melting the charge to provide a melt at the base of the furnace having the composition of the fibres, removing melt from the base of the furnace and fiberising the melt, characterised in that at least one quarter of the aluminium in the charge is introduced as particulate alumina-containing mineral in briquettes and the particulate alumina-containing mineral contains from 0.5 to 10 wt.% metallic aluminium, 50 to 90 wt.% alumina Al 2 0 3 and 0 to 49.5 wt.% other materials. The invention also provides novel briquettes suitable for use in the manufacture of high aluminium MMVF (e.g. MMVF containing at least 14% aluminium) which contain at least 5% (by weight of the briquette) particulate alumina- containing mineral containing 0.5 to 10 wt.% metallic aluminium, 50 to 90 wt.% alumina Al 2 0 3 and 0 to 49.5 wt.% other materials. In particularly preferred aspects of the invention the particulate alumina-containing mineral has a controlled particle size distribution. In particular the particulate alumina-containing mineral has size 90% by weight below lmm, preferably 90% by weight below 200 microns. Preferably the average particle size is from 10 to 100 microns, for instance 20 to 30 microns. We find that the choice of the defined specific particulate alumina-containing minerals, out of all the general ranges of virgin and waste aluminium-containing materials which are known, gives particular benefits in processes where briquettes are melted in a shaft furnace . The presence of the defined proportion of metallic aluminium gives benefits in the melting process, since it is oxidised exothermically in the shaft furnace. This contributes energy towards melting of the other components, such as alumina Al 2 0 3 , and can reduce the fuel requirements. The defined maximum proportion of alumina Al 2 0 3 in the mineral lowers the melting point of the mineral in comparison with bauxite and very high Al 2 0 3 wastes such as filter dust and thus melts more easily and more completely in the residence time available. The preferred small particle size materials also contribute further to the melting benefits. We also find that the use of the specified high aluminium, alumina-containing mineral, especially when provided in a form which has the preferred particle size distribution defined above, gives improved strength to the briquettes . The high aluminium mineral must contain from 0.5 to 10 wt.% metallic aluminium. Preferably it contains from 2 to 6 wt . % , more preferably below 5 wt.%, metallic aluminium. The high aluminium mineral contains from 50 to 90 wt.% alumina A1 2 0 3 , preferably below 85 wt.%, more preferably from 60 to 72 wt.%. Contents of metallic aluminium and alumina (and other components) are on a dry basis and can be determined using standard methods. For instance, content of metallic aluminium can be determined by reacting the material with a strong acid, such as hydrochloric acid. The amount of metallic aluminium can be determined from the amount of hydrogen gas released. The alumina-containing mineral contains 0 to 49.5 wt.% other materials, generally at least 5 wt.%. Correct choice of these other materials can increase the usefulness of the high aluminium material in the briquettes. In particular, certain other materials can act as fluxing agents which improve the melting ability of the material in the briquettes. In particular it is preferred that the other materials include at least 5 wt.% Si0 2 and MgO. For instance, the total amount of these oxides is generally from 3 to 35%, preferably 10 to 25%. Preferred amounts of Si0 2 are 3 to 20%, more preferably 6 to 15%. Preferred amounts of MgO are 3 to 15%, more preferably 5 to 10%. Preferably the alumina-containing mineral contains Fe 2 0 3 in an amount of from 0.5 to 10 wt.%, more preferably 1 to 6 wt.%. In particular it is preferred that the alumina- containing mineral material contains oxides of corundum, spinel and mullite. Preferably the crystals of these oxides in the mineral meet the particle size ranges discussed above . Any alumina-containing mineral material which meets the requirements set out above may be used. Preferably it is a waste material. In particular, wastes from secondary production of aluminium, e.g. processes of casting aluminium, are useful. These are often described generically as "aluminium dross" or ' "aluminium oxide dross" . In particular, the aluminium casting process provides a specific alumina-rich waste material described commonly as "alu-dross" . This tends to contain significant proportions of metallic aluminium and is thus treated in order to retrieve the metallic aluminium. The alu-dross is generally crushed, milled and sieved. This produces some aluminium for re-sale and an aluminium-rich fraction which is sent to a furnace for re-use. As a by-product an alumina-rich powder is also produced. This powder can usefully be incorporated into briquettes for use in the invention and is described herein as "crushed alu-dross". This alumina-rich powder generated from treatment of alu- dross (crushed alu-dross) may contain levels of halogen materials (by weight) of for instance 1 to 10%, preferably 1 to 8%. Halogens include in particular fluoride and chloride . The aluminium-rich fraction, optionally together with other aluminium-containing waste materials, is subjected to re-melting in a furnace. This may be a rotating furnace or kiln. The aluminium waste may be subjected to plasma heating. A conventional furnace may be used. Salt is usually added to the furnace in order to reduce the surface tension of the aluminium and reduce oxidation. This process produces an aluminium fraction for resale, more alu-dross and a salt slag material. The salt slag can be subjected to a wet chemical process (involving water washing and high temperature treatment) which produces a salt fraction, which is recycled to the furnace, and a further alumina- rich powder. This second alumina-rich powder can also be usefully incorporated in briquettes in the invention and is described herein as "treated aluminium salt slag" . This product tends to have lower content of halogen materials (e.g. fluoride) than the alumina-rich powder produced by treatment of alu-dross (crushed alu-dross) . Its content of halogen (by weight) tends to be from 0 to 5%, often at least 0.5 or 1%, and is preferably not more than 3%. The particular alumina-rich powder which is chosen will depend upon the requirements of the process. Halogen- containing alumina-rich powders can be advantageous, as discussed in our co-pending application number , reference PRL03866WO, filed today and claiming priority from European application number 97309675.3. Powders containing 1 to 3% halogen, e.g. treated aluminium salt slag, are preferred in the invention. Both crushed alu-dross and treated aluminium salt slag have the advantage that as received they have particle size in or close to the preferred ranges discussed above. Thus they may be used for incorporation into briquettes without further size reduction or, if the distribution is not exactly as discussed above, after selection of appropriate fractions. Thus they have the further advantage over bauxite that extensive grinding and crushing are not required. Some of the alumina-rich powders are used in the cement industry and are sold under the tradenames Oxiton, Valoxy and Oxidur. These may be used in the invention. However, large proportions of the alumina-rich powder are presently sent to landfill and an advantage of the invention is that it provides further uses for these materials (as well as the technical benefits obtained by using them) . The fibres produced in the invention have high aluminium content (measured by weight of A1 2 0 3 ) , namely at least 14%, preferably at least 15%, more preferably at least 16% and in particular at least 18%. Generally the amount of aluminium is not more than 35%, preferably not more than 30%, more preferably not more than 26 or 23%. In general the fibres and the melt from which they are formed have an analysis (measured as % by weight of oxides) of other elements within the various ranges defined by the following normal and preferred lower and upper limits : Si0 2 : at least 30, 32, 35 or 37; not more than 51, 48, 45 or 43 CaO: at least 8 or 10; not more than 30, 25 or 20 MgO: at least 2 or 5; not more than 25, 20 or 15 FeO (including Fe 2 0 3 ) : at least 2 or 5; not more than 15, 12 or 10 FeO+MgO: at least 10, 12 or 15; not more than 30, 25 or 20 Na 2 0+K 2 0: zero or at least 1; not more than 10 CaO+Na 2 0+K 2 0 : at least 10 or 15; not more than 30 or 25 Ti0 2 : zero or at least 1; not more than 6, 4 or 2 Ti0 2 +FeO: at least 4 or 6; not more than 18 or 12 B 2 0 3 : zero or at least 1; not more than 5 or 3 P 2 0 5 : zero or at least 1 ; not more than 8 or 5 Others: zero or at least 1; not more than 8 or 5 In the invention it is preferred that the amount of iron in the fibres is from 2 to 15%, preferably 5 to 12%. Shaft furnaces such as cupola furnaces tend to have a reducing atmosphere, which can result in reduction of iron oxides and formation of metallic iron. This is not incorporated into the melt and fibres and must be removed from the furnace. Thus the conditions in the furnace must be carefully controlled to avoid excess reduction of iron. It is surprising that the inclusion of metallic aluminium is advantageous in such processes, since it is oxidised in the furnace and might be expected to increase reduction of iron. However, we find that it is possible in the invention to produce final product fibres having significant levels of iron oxide. The invention is of particular value in the production of fibres which can be shown to be soluble in physiological saline. Suitable high aluminium, biologically soluble, fibres which can advantageously be made in the present invention are described in W096/14454 and W096/14274. Others are described in WO97/29057, DE-U-2970027 and WO97/30002. Reference should be made to each of these. The fibres preferably have an adequate solubility in lung fluids as shown in vivo tests or in vitro tests, typically conducted in physiological saline buffered to about pH 4.5. Suitable solubilities are described in W096/14454. Usually the rate of dissolution is at least 10 or 20nm per day in that saline. The fibres preferably have sintering temperature above 800 °C, more preferably above 1000 °C. The melt preferably has a viscosity at fibre forming temperature of 5 to 100 poise, preferably 10 to 70 poise at 1400°C. It is essential in the invention that the furnace is a shaft furnace in which a self-supporting stack of mineral material is heated and melt drains to the base of the stack. Usually it forms a pool from which it is run off to the fibre forming process. In some cases the melt can be run from the base of the stack into another chamber where it collects as a pool and from which it is run off to the fibre-forming process. The preferred type of shaft furnace is a cupola. It is also essential in the invention that the charge includes briquettes. The briquettes are made in known manner by molding a mix of the desired particulate materials (including the high aluminium material) and a binder into the desired briquette shape and curing the binder. The binder may be a hydraulic binder, that is one which is activated by water, for instance Portland cement. Other hydraulic binders can be used as partial or complete replacement for the cement and examples include lime, blast furnace slag powder (JP-A-51075711) and certain other slags and even cement kiln dust and ground MMVF shot (US 4662941 and US 4724295) . Alternative binders include clay. The briquettes may also be formed with an organic binder such as molasses, for instance as described in W095/34514; such briquettes are described herein as formstones . At least one quarter of the aluminium in the fibres is provided by the defined high-aluminium mineral incorporated into the briquettes. Preferably at least 50%, more preferably at least 75% and most preferably substantially all of the aluminium in the fibres is provided by the defined high-aluminium material. Generally at least 20 to 25%, preferably at least 30% of the charge (by weight) is provided by briquettes. In some processes higher amounts, e.g. 45 to 55%, are preferred and amounts above 75% or even above 80% are sometimes preferred. The invention is especially beneficial in processes where a significant (e.g. above 25%) proportion of the charge is in the form of briquettes. The briquettes generally contain (by weight) at least 5% of the defined aluminium-containing mineral, preferably at least 10 or 15%. They may contain more than 20% but generally do not contain more than 45 or 50% of the defined aluminium-containing material. The other materials in the briquettes and in the rest of the charge may be any suitable virgin or waste materials . Other suitable wastes that can be used in the invention include slags from the metallurgical industry, especially steelmaking slags such as converter slags or EAF slags, and slags from the ferro-alloy industry such as ferro-chromium, ferro-manganese or ferro-silica slags; slags and residues from the primary production of aluminium such as spent aluminium pot lining or red mud; dried or wet sludge from the paper industry; sewage sludge; melasse; bleaching clay; residues from the incineration of household and industrial wastes, especially slags or filter ashes from the incineration of municipal solid wastes; glass waste (or slags) from the vitrification of other waste products; glass cutlets; waste products from the mining industry, especially minestone from the excavation of coal; residues from the incineration of fossil fuel, especially from the combustion of coke at power plants; spent abrasive sand; spent moulding sand from iron and steel casing; waste sieving sand; glass reinforced plastic; and fines and breakage waste from the ceramic and brick industry. Toxic virgin rock can also be used as waste. Because the invention may advantageously use waste materials, which may be of variable content, it can be desirable to monitor the melt or the fibre properties and to change the process conditions as necessary in order to maintain uniform production. Preferably this is done as described in our application number , reference PRL03858WO, filed even date herewith and claiming priority from European application no. 97309674.6. The MMV fibres may be made from the fibre- forming mineral melt in conventional manner. Generally they are made by a centrifugal fibre- forming process. For instance the fibres may be formed by a spinning cup process in which they are thrown outwardly through perforations in a spinning cup, or melt may be thrown off a rotating disc and fibre formation may be promoted by blasting jets of gas through the melt. Preferably fibre formation is conducted by pouring the melt onto the first rotor in a cascade spinner. Preferably the melt is poured onto the first of a set of two, three or four rotors, each of which rotates about a substantially horizontal axis whereby melt on the first rotor is primarily thrown onto the second (lower) rotor although some may be thrown off the first rotor as fibres, and melt on the second rotor is thrown off as fibres although some may be thrown towards the third (lower) rotor, and so forth. The following are Examples. Each of these describes a charge for a cupola furnace and the analysis of the subsequent melt which can be fiberised, for instance using a cascade spinner. Example 1 Cement briquettes with treated aluminium salt slag Cement Briquettes Treated aluminium salt slag: 16.5%, Cement: 14.5%, Process waste wool: 37%, Process waste slag: 21%, Ladle slag: 4.5%, bottom slag: 3.5%, Bauxite: 3% These cement briquettes had the advantage over "normal" cement briquettes in their higher resistance to the mechanical stress from transportation and logistics with reduced loss to fines and improved stability in the furnace . Charge to Furnace Cement Briquette: 50%, basalt: 50% Mel t composi tion from furnace Example 2 Clav briquettes with treated aluminium salt slag Clay briquettes Treated aluminium salt slag: 8%, Clay: 50%, Olivine sand: 4%, Iron ore: 2%, process waste wool: 32%, Other process waste: 4% Cement .briquet es Treated aluminium salt slag: 40%, Ladle slag: 51%, Cement: 9% Charge to Furnace Clay briquettes: 86%, Cement briquettes: 6%, Converter slag: 6%, Lump process slag: 2% The total content of treated aluminium salt slag in the charge is 9.3%. Melt composi tion from furnace Compared to normal conditions the coke consumption was reduced by 1.5% (from 13.2 to 11.7% when the clay briquettes with treated aluminium salt slag were used. This was accompanied by an increased melt temperature (from 1495-1510°C to 1526-1530°C) . Example 3 Formstones with treated aluminium salt slag Formstones Treated aluminium salt slag: 19%, Lime: 3%, Molasses: 9%, process waste: 64%, Iron ore: 5% Charge to furnace Formstones: 31%, Diabase: 47%, Blast furnace slag: 16%, Dolomite: 6% Melt composi tion from furnace Replacing the bauxite lumps normally used with 20% of the formstones saved 1% (from 12.8 to 11.8%) coke.

Claims: CLAIMS 1. A process of producing man-made vitreous fibres having a composition which contains at least 14% aluminium measured as weight Al 2 0 3 based on oxides, comprising providing in a shaft furnace a self-supporting stack of a mineral charge which includes briquettes, melting the charge to provide a melt at the base of the furnace having the composition of the fibres, removing melt from the base of the furnace and fiberising the melt, characterised in that at least one quarter of the aluminium in the charge is introduced as particulate alumina-containing mineral in briquettes and the particulate alumina-containing mineral contains from 0.5 to 10 wt.% metallic aluminium, 50 to 90 wt.% alumina A1 2 0 3 and 0 to 49.5 wt.% other materials. 2. A process according to claim 1 in which the particulate alumina-containing mineral has size 90% by weight below 200 microns. 3. A process according to claim 1 or claim 2 in which the alumina-containing mineral has a content of metallic aluminium from 2 to 6 wt . % . 4. A process according to any preceding claim in which the alumina-containing mineral has a content of alumina A1 2 0 3 from 60 to 72 wt.%. 5. A process according to any preceding claim in which the alumina-containing mineral contains from 3 to 20 wt.% Si0 2 and from 3 to 15 wt.% MgO. 6. A process according to any preceding claim in which the alumina-containing material is crushed alu-dross. 7. A process according to any of claims 1 to 5 in which the alumina-containing mineral is treated aluminium salt slag. 8. A process according to claim 7 in which the treated aluminium salt slag has a content of halogen, preferably fluorine, of from 1 to 4% by weight. 9. A process according to any preceding claim in which the fibres have aluminium content measured by weight of A1 2 0 3 from 18 to 30%. 10. A process according to any preceding claim in which the fibres have iron content measured by weight of FeO of from 5 to 12%. 11. A process according to any preceding claim in which the furnace is a cupola f rnace. 12. A process according to any preceding claim in which at least 25% of the mineral charge is formed by briquettes. 13. A briquette suitable for the production of man-made vitreous fibres containing at least 5%, by weight of the briquette, of a particulate alumina-containing mineral containing 0.5 to 10 wt.% metallic aluminium, 50 to 90 wt.% alumina Al 2 0 3 and 0 to 49.5 wt.% other materials. 14. A briquette according to claim 13 which contains at least 10%, by weight of the briquette, of the particulate alumina-containing mineral. 15. A briquette according to claim 13 or claim 14 having any of the additional features set out in claims 2 to 8.

Inventors: NYKYEL GUIDO, RANLOV JENS, WITTKAMP PETER, NYKYEL, GUIDO, RANLOV, JENS, WITTKAMP, PETER
Applicants: NYKYEL GUIDO, RANLOV JENS, ROCKWOOL INT, WITTKAMP PETER, NYKYEL, GUIDO, RANLOV, JENS, ROCKWOOL INTERNATIONAL A/S, WITTKAMP, PETER
Application year: 1998
Main classification: C03C1
IPC codes: C03C1, C03C13