Process for recycling residual mineral material

Abstract

The present invention relates to a process for preparing a mineral melt in a cupola furnace comprising a plasma torch that heats the mineral material and waste or recycled mineral material to form the melt, wherein the waste or recycled mineral material is introduced into the furnace through a waste inlet located in the side wall of the hot zone of the furnace.

Description

Process for recycling residual mineral material Field of invention The present invention relates to a process for preparing a mineral melt in a cupola furnace comprising a plasma torch that heats mineral material and recycled or residual mineral material to form the melt, wherein the recycled or residual mineral material is introduced into the furnace through a waste inlet located in the side wall of the hot zone of the furnace. Background It is known that methods for preparing a mineral melt for the production of artificial mineral fibers (AMFs), such as fiberglass or rock fiber, are carried out in shaft furnaces, such as cupola furnaces. These methods involve heating the mineral material to form the mineral melt. Heating can be provided by any means, but traditionally it is carried out in the presence of coke and an oxygen-containing gas, or by passing an electric current between two or more electrodes, e.g., graphite or molybdenum electrodes. The mineral material can also be melted using a plasma torch. The use of plasma as a heat source is advantageous because it can minimize the production of harmful gases, such as CO2, NOx, and H2, which would otherwise be present in the furnace exhaust gas. The spinning processes used to form MMVF inevitably produce various byproducts that constitute residual mineral material. This residual mineral material can be in the form of fine powders, such as those formed when MMVFs are cut, or it can be fibrous in nature. Additionally, MMVF products that have reached the end of their useful life, such as MMVF insulation removed during building renovations, are also considered residual mineral material. Residual mineral material may include a binder used during the MMVF manufacturing process. The waste may also be in a cured or uncured state. As an alternative to landfill disposal, residual mineral material can be recycled by combining it with other (conventional) mineral material and heating it to form a mineral melt. This mineral melt can then be spun to form new MMVF products. Despite the benefits of recycling residual mineral material, it cannot simply be combined with conventional mineral material and added to the furnace in the usual way. Direct introduction of residual mineral material into a blast furnace, such as placing it on top of the ore pile, is unacceptable. Due to the various geometries of the residual mineral material, e.g., fine powders and fibrous material, and its insulating properties, it can clog or smother the furnace by preventing residual gases from rising through the ore pile, restricting the flow of oxygen through the furnace's tuyeres and oxygen injection openings, or restricting the flow of plasma from a plasma torch into the furnace. This greatly reduces furnace efficiency and can also cause the melting process to fail. Residual mineral material can be processed into briquettes that can be added to a blast furnace along with conventional mineral material. However, the briquetting process requires the addition of a binder to the residual mineral molten material, which exacerbates environmental problems. The use of these briquettes is also unattractive, as it increases the complexity and cost of the recycling process and is therefore not viable on an industrial scale. Application WO 90 / 07470 describes a method for preparing a melt for the production of mineral wool using waste wool, in which heat is provided by a plasma torch. The waste wool is fed into a connecting pipe by a screw conveyor, and the connecting pipe carries the plasma jet to the cupola furnace. In this method, the waste wool is added to the plasma outside the cupola furnace. One problem with this method is that the feed pipe housing the screw conveyor, as well as the connecting pipe, are prone to clogging with slag (components of the waste wool), a problem exacerbated when the feed pipe and connecting pipe are water-cooled to extend the device's lifespan.A preferred embodiment attempts to reduce the impact of feed pipe clogging by gradually increasing the diameter of the feed pipe as seen in the direction towards the connecting pipe; however, the connecting pipe remains prone to clogging. Application WO2012157432 A1 discloses a glass melting method using a tank furnace, where the raw material is fed into the top of the furnace through a burner, after which the raw material passes through a hot zone at the bottom of the furnace heated by a plasma. US patent 5069429 A discloses a furnace for melting metal compounds that, in the broadest sense, could be considered suitable for melting glass. The furnace has a hot zone at the bottom that is heated by a plasma torch. The raw material is fed into the top of the furnace. Therefore, there is a need for an improved method to form a melt suitable for use in mineral fiber formation, a method that utilizes a significant proportion of residual mineral material. Summary of the invention In a first aspect of the invention, a process is provided for preparing a mineral melt in a cupola furnace, wherein the mineral melt is for the formation of mineral wool, and the cupola furnace comprises (i) a hot zone at the base of the furnace; (ii) a melt outlet in the hot zone; (iii) at least one plasma torch that provides plasma heating to the hot zone by means of a plasma jet that defines a cavity that is substantially devoid of solid material within the hot zone; and (iv) a waste inlet located in the side wall of the hot zone of the furnace and configured to supply waste mineral material to the cavity defined by the plasma jet, where the mineral material and residual mineral material supplied to the furnace are melted to form the mineral melt. In a second aspect of the invention, a process for manufacturing artificial vitreous fibers (MMVF) is provided, comprising the steps of (i) forming a mineral melt using a process as defined in the first aspect of the invention, or any embodiment thereof; (ii) to fibrose the mineral melt by an internal or external spinning process, preferably using a cascade spinneret; and (iii) collect the fibers formed. In a third aspect of the invention, a cupola furnace is provided for the preparation of a mineral melt according to a process as defined in the first aspect of the invention, or any embodiment thereof. Brief description of the drawings Figure 1 is a schematic representation of a section of a cupola furnace configuration that can be used to implement the invention. Detailed description The invention is based on the unexpected finding that a waste inlet located in the wall of the hot zone of a cupola furnace can efficiently and effectively supply waste mineral material to a plasma jet where it melts to form a mineral melt, without the aforementioned drawbacks. Cupola furnaces heated using plasma typically comprise a hot zone in the lower portion of the furnace, also called the furnace base. One or more plasma torches can heat the hot zone to a level sufficient to melt mineral material (a mineral charge). The mineral melt thus forms a pool at the base of the hot zone, in the spaces between the structure supporting the pile of mineral material (usually the mineral material itself or coke). Once a certain volume of mineral melt has been produced, it can be removed from the furnace through some form of melt outlet, such as a siphon. Once removed from the furnace, the mineral melt can be used to form artificial glass fiber (MMVF) by any suitable spinning method. Plasma torches generate thermal plasma using direct current (DC), alternating current (AC), radio frequency (RF), and other discharges. Thermal plasmas provide heat, which in DC plasma torches is produced by sending an electric arc between two electrodes, through which a carrier gas is passed within a constricted orifice. This raises the temperature of the gas to the point where it enters a fourth state of matter, i.e., plasma. Plasma torches can be of the transferred or non-transferred type. In non-transferred DC plasma torches, the electrodes are inside the torch housing. In a transferred plasma torch, one electrode is located outside the torch housing, allowing the arc to form outside the plasma torch and at a greater distance. It is preferable that the plasma torch of the present invention be a non-transferred plasma torch.The most preferable option is a direct current non-transferred plasma torch. Plasma torches can use a variety of carrier gases, such as oxygen, nitrogen, argon, helium, air, hydrogen, water vapor, or mixtures thereof. One or more plasma torches are typically mounted on an external side of the furnace, so that the generated thermal plasma, commonly referred to as the plasma jet, penetrates the furnace's hot zone. The plasma jet can reach temperatures up to 7000 °C, which is sufficient to melt the ore material in its vicinity. As the solid ore material melts (and is subsequently removed from the furnace through the melt outlet), the ore stockpile descends the furnace to replace it. In practice, the plasma jet has been found to define a cavity (a three-dimensional shape) within the furnace that is substantially devoid of solid material, such as ore, residual ore material, and coke.The temperature and environment within the plasma jet are such that the ore in the furnace will typically melt before entering the defined cavity; however, that cavity may contain some solid material. For example, a section of the ore pile may break off from the bulk material and fall into the plasma jet. Alternatively, due to the melting process, the pile may suddenly and partially collapse, allowing solid ore to enter the plasma jet. This solid material also includes residual ore from the waste inlet, as explained below. In either case, any solid material within the cavity would be transient, as the plasma jet would quickly melt it. Ideally, the plasma jet should define a cavity within the hot zone that is free of solid material. In other words, no mineral material or residual mineral material should enter that cavity. The term "residual mineral material" refers to the material mentioned above and includes various byproducts of the MMVF (mineral wool fiberglass) manufacturing process, which are generally referred to as mineral wool waste. These byproducts include a fine powder material formed in connection with cutting, e.g., edge trimming, of mineral wool mats to form mineral wool panels of the desired dimensions. It also includes recycled mineral material, such as MMVF insulation removed from buildings during renovation, which might otherwise end up in a landfill. Residual mineral material may also include additives, such as binders and hydrophilic or hydrophobic compounds, that were added during the manufacturing process. The cupola furnace includes a waste inlet located in the side wall of the furnace's hot zone. The waste inlet is essentially an opening in the furnace wall. It is configured to supply waste mineral material to the cavity defined by the plasma jet. In essence, the waste inlet can supply waste mineral wool directly to the plasma jet. It will be noted that, due to the environment surrounding the plasma jet, the waste mineral material may melt before entering the jet. This can depend on the quantity or rate at which the waste mineral material is introduced into the furnace through the waste inlet. Once melted, it will become part of the mineral melt at the base of the hot zone and can be removed through the melt outlet. The residual mineral material can be introduced through the waste inlet and into the kiln by any suitable means. Such a means can be (a) a screw conveyor; or (b) pneumatic tube conveying, preferably where the pneumatic tube conveying uses recycled waste gas from the cupola furnace or gaseous nitrogen (N2) or any other gas that does not contain oxygen. It is preferable that the residual mineral material be introduced through the waste inlet using a screw conveyor. The rate of adding residual mineral material to the cupola furnace can depend on the furnace size and melt production rate, the number of plasma torches, the number of waste inlets, and their size and position within the furnace. Ideally, the amount of residual mineral material used in the process to prepare a mineral melt is 1 to 40%, preferably 5 to 35%, and more preferably 10 to 20% of the total weight of the combined residual and mineral material added to the furnace. The formation of a mineral melt in a cupola furnace is usually a continuous process, and therefore the ratio of residual mineral material to the combined residual and mineral material can be measured over a certain period. This is because the residual and / or mineral material can be added to the cupola furnace in a fractional manner. To ensure an adequate rate of addition of residual mineral material to the cupola furnace, it is preferable that each waste inlet located in the furnace wall have an area of ​​3 cm² to 320 cm², preferably 20 cm² to 180 cm², and more preferably 50 cm² to 115 cm². This is roughly equivalent to a circular hole with a diameter of 2 cm to 20 cm, preferably 5 cm to 15 cm, and more preferably 5 cm to 12 cm. Ideally, the waste inlet is located on the side wall of the hot zone of the furnace, above the point where the plasma jet enters the furnace. The advantage of this is that the waste mineral material can easily move into the cavity defined by the plasma jet due to gravity. As such, this is a highly preferred feature of the invention. The distance between the edge of the orifice through which the plasma jet enters the furnace and the edge of the waste inlet must be sufficient to ensure that the residual mineral material can be melted and incorporated into the mineral melt. This distance can range from 1 cm to 30 cm, preferably from 5 cm to 20 cm, and more preferably from 7 cm to 12 cm. Frequently, this distance is determined by the dimensions of the water-cooling jackets for both the waste inlet and the plasma jet. Due to the proximity of the waste inlet to the plasma jet, it is preferable that the waste inlet and the means by which the waste mineral material can be fed through the waste inlet (e.g., a screw conveyor or pneumatic tube conveyor) be made of heat-resistant material and / or equipped with a cooling mechanism. The hot zone of a cupola furnace typically comprises a layer of refractory bricks. Therefore, the waste inlet can be an opening in these refractory bricks that provides adequate heat resistance. In addition, plasma torches are typically water-cooled. The plasma torch's water-cooling system can be extended to also cool the means by which the waste mineral material can be fed through the waste inlet.This can extend the furnace's lifespan and minimize downtime for maintenance, thereby increasing process efficiency. Due to the diameter of typical cupola furnaces, it may be preferable to have two to five plasma torches, preferably three or four, especially if the plasma torch is the primary or sole means of heating the furnace. The plasma torches should be positioned substantially horizontally around the furnace's perimeter and substantially equidistant from one another. This means that the plasma jet from each torch extends toward the center of the cupola furnace. To maximize the rate at which residual mineral material can be added to the furnace, each plasma torch should have an associated waste inlet. While the waste mineral material can take various forms, more uniform waste may be preferable. Therefore, it can be ground before being fed into the cupola furnace through the waste inlet. A rod mill is preferable. In this case, it is preferable for the waste mineral material to be ground to a density of 500 kg / m³ to 1500 kg / m³, such as 700 kg / m³ to 1250 kg / m³, or, for example, 800 kg / m³ to 1000 kg / m³. Grinding it to these densities means it can be more easily added to the cupola furnace using a screw conveyor or pneumatic tube conveyor. The claimed process is particularly suitable in conjunction with the process as set forth in application WO 2022 / 106592. That process is for the production of a mineral melt suitable for use in the formation of MMV fibers, such as fiberglass or rock fiber, a process that minimizes the amount of NOx and H2 produced in the waste gas of a cupola furnace even when a plasma torch is used. Such advantages can be achieved by supplying more than 50% of the heating energy to the cupola furnace using a plasma torch, wherein the plasma torch uses nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), or a mixture thereof as the carrier gas, and water is excluded from any area of ​​the cupola furnace above 750 °C. As is known, cupola furnaces typically comprise a range of temperature zones, including a hot zone, an oxidation zone, a reduction zone, and a preheating zone. In view of the above, in a specific characteristic of the present process (i) the furnace is equipped with at least one tuyere and / or oxygen injection opening that provides a source of oxygen in the oxidation zone of the cupola furnace; (ii) the at least one plasma torch uses as a carrier gas N2, CO, CO2 or a mixture thereof, optionally wherein the enthalpy of the carrier gas is from 2.0 to 6.0 kWh / Nm3, such as from 3.0 to 5.0 kWh / Nm3, and preferably wherein the carrier gas is nitrogen (N2); (iii) more than 50% of the furnace heating energy is provided by the plasma torch; (iv) the temperature in the oxidation zone is below 1400 °C; (v) the temperature in the hot zone is higher than the temperature in the oxidation zone; and / or (vi) Water is substantially excluded from any area of ​​the furnace where the temperature is above 750 °C. It is preferable to include all features (i) to (vi) to minimize the amount of NOx and H2 produced in the exhaust gas. When this is a process objective, (I) more than 60%, preferably more than 70%, more preferably more than 80%, even more preferably more than 90%, most preferably all of the cupola furnace heating energy is provided by at least one plasma torch; and / or (II) wherein heating is provided in the hot zone solely by at least one plasma torch. Furthermore, NOx can be significantly reduced by excluding oxygen from the nitrogen-containing zones of a cupola furnace at temperatures of 1400 °C or higher. To help minimize NOx production, the carrier gas should contain only trace amounts of oxygen. This means that the carrier gas should contain less than 5% oxygen by weight, such as less than 2%, and preferably less than 0.8% by weight, relative to the total weight of the carrier gas. Ideally, the carrier gas is oxygen-free. This means that only trace amounts of oxygen are present. NOx can be derived from nitrogen and oxygen at higher temperatures. To reduce NOx formation, the temperature in the oxidation zone of the cupola furnace should be between 600 °C and 1400 °C. To further reduce NOx formation, it is preferable that the oxidation zone temperature be between 600 °C and 1300 °C, more preferably between 600 °C and 1200 °C, even more preferably between 600 °C and 1100 °C, especially between 600 °C and 1000 °C, most preferably between 600 °C and 900 °C, and particularly between 600 °C and less than 850 °C. The waste gas from cupola furnaces heated using plasma torches may comprise N2, CO, CO2, NOx, and H2, each of which is a component of the waste gas. The waste gas may also comprise additional components, such as water and particulate matter. In one particular application, the waste gas, in whole or in part, may be used as the carrier gas for one or more of the plasma torches. The components of the waste gas may be separated prior to their use as carrier gas. The waste gas components may be separated from each other, or a combination of two or more components may be separated from the other components. This means that the carrier gas may comprise at least one component of the waste gas, such as one, two, three, four, five, or more components. It is preferable that the carrier gas comprise the waste gas components N2, CO, CO2, or a combination thereof.Alternatively, the carrier gas may comprise a component of the waste gas, such as N2, CO or CO2. One or more components of the waste gas may undergo waste gas cleaning before use in the carrier gas. It is preferable that the waste gas cleaning remove suspended particles and / or water from the waste gas. Waste gas cleaning may be performed on the waste gas as a whole, or on at least one component of it once it has been separated from the rest of the waste gas. The carrier gas may consist of the waste gas, or at least one component of the waste gas. Alternatively, it may comprise the waste gas, or at least one component of the waste gas. In the latter case, additional gas that was not part of the waste gas may be added to the carrier gas before use. In this case, the carrier gas is "filled" with additional gas. The process can be used to form any mineral melt suitable for mineral wool production. In particular, the process is especially advantageous in the formation of a mineral melt where the resulting mineral melt or mineral wool-based melts have a chemical analysis (measured as weight percent oxides) of elements within the various ranges defined by the following normal and preferred lower and upper limits. SiO2 35-50, preferably 38-48, more preferably 33-44; Al2O3 12-30, preferably 15-28, more preferably 16-24; TiO2 up to 2 Fe2O3 2-12 CaO 5-30, preferably 8-20 MgO 0-15, preferably 1-12 Na2O 0-15 K2O 0-15 P2O5 0-3 MnO 0-3 B2O3 0-3. In this case, it is preferable that, when the melt is to be transformed into MMVF, the proportion of Fe(2+) in the mineral melt be more than 80% based on the total Fe, preferably at least 90%, more preferably at least 95%, and most preferably at least 97% based on the total Fe. In such cases, it is preferable that the MMVF be manufactured using a cascade spinneret. Further details of these example mineral melts can be found in application WO 2012 / 140173. As is conventional in the art, references herein to Fe₂O₃ in a mineral melt or fiber composition refer to the total amount of iron (calculated in terms of Fe₂O₃) in that melt or composition, regardless of the amount of each oxidation state of iron present in the composition. In the preceding example of the mineral melt and the resulting fibers, the amount of iron in the mineral melt is preferably 2 to 15% by weight, and more preferably 5 to 12% by weight. Cupola furnaces tend to have a reducing atmosphere, which can result in the reduction of iron oxides and the formation of metallic iron. Preferably, the metallic iron is not incorporated into the mineral melt or the fibers and should be removed from the furnace. Thus, the conditions in the furnace can be carefully controlled to avoid excessive iron reduction.However, we observed that it is possible to produce final product fibers that have significant levels of iron oxide. The process of the invention can be used in the formation of fibers that can be shown to be soluble in physiological saline solution. Suitable high-aluminum, biologically soluble fibers that can be advantageously manufactured using the process of the present invention are described in applications WO96 / 14454 and WO96 / 14274, and others are described in applications WO97 / 29057, DE-U-2970027, and WO97 / 30002. Such fibers preferably have sufficient solubility in pulmonary fluids, as shown in in vivo or in vitro tests, typically performed in buffered physiological saline solution at a pH of approximately 4.5. Suitable solubilities are described in application WO96 / 14454. Usually, the dissolution rate is at least 10 or 20 nm per day in such saline solution. The fibers preferably have a sintering temperature above 800 °C, more preferably above 1000 °C. The melt preferably has a viscosity at the fiber formation temperature of 5 to 100 poises, preferably 10 to 70 poises at 1400 °C. Further embodiments of this example can be found in application WO99 / 28252. MMVFs can be formed as a bonded band comprising MMVFs as described above, or MMVFs manufactured according to the process described above, and a cured binder composition. Preferably, the mineral melt in this particular example has a viscosity in the range of 10 to 30 poises at 1400 °C, more preferably in the range of 20 to 25 poises. One advantage of choosing these viscosities is that the resulting MMVFs have a smaller diameter than if the melt viscosity were higher. Furthermore, it is possible to use the melt at a lower temperature to achieve the required operating viscosities. This saves energy, as the melt can be used at a lower temperature. It also reduces wear on rotors used to produce fibers, since a lower-temperature melt causes less wear. Further details of this example mineral melt can be found in application WO 2015 / 055758. The viscosity of the melt can be determined according to ASTM C 965-96. The raw materials (mineral material) may be in briquette form. Briquettes are manufactured in a known manner by molding a mixture of the desired particle size materials and a binder into the desired briquette shape and curing the binder. The binder may be a hydraulic binder, i.e., one that is activated by water, for example, Portland cement. Other hydraulic binders may be used as a partial or total replacement for cement, and examples include lime, blast furnace slag powder and certain other slags, and even cement kiln powder and crushed MMVF shot (patents JP-A-51075711, US-4,662,941 and US-4,724,295). Alternative binders include clay. Briquettes can also be formed with an organic binder such as molasses, for example, as described in application WO 95 / 34514. Such briquettes can be described as shaped pieces. The mineral melt prepared by the process of the invention may be suitable for the production of mineral wool or MMVF, such as glass fibers or rock fibers. It is preferable that the mineral melt formed be suitable for use in the formation of MMVF. Therefore, in the second aspect of the invention, a process for manufacturing MMVF is provided, comprising the steps of (i) forming a melt using a process as defined in this document; (ii) to fiberize the melt by an internal or external spinning process; preferably using a cascade spinning machine; and (iii) collect the fibers formed. Fibers, particularly MMVFs, can be manufactured from mineral melt in a conventional manner. They are generally produced using a centrifugal fiber-forming process. For example, fibers can be formed by a spinning cup process in which they are thrown out through perforations in a spinning cup, or the mineral melt can be thrown from a rotating disc and fiber formation can be promoted by jets of pressurized gas through the mineral melt. Fiber formation can also be carried out by pouring the mineral melt onto the first rotor in a cascade spinning machine.In this case, it is preferable that the mineral melt be poured onto the first of a set of two, three, four, or even more rotors, each rotating around a substantially horizontal axis. The mineral melt in the first rotor is primarily cast onto the second (lower) rotor, although some may be cast from the first rotor as fibers, and the mineral melt in the second rotor is cast as fibers, although some may be cast onto the third (lower) rotor, and so on. Generally, it is preferable for the spinning process to utilize a cascade spinning frame. Those skilled in the art know the required properties of a mineral melt to be used in each spinning method, and the composition of the mineral melt can be adjusted to provide these properties.For example, experts in the technique can select mineral materials to be added to the cupola furnace to produce a specific composition of mineral melt, which will be spun using a particular spinning process. During the fiberization process, the melt is transformed into a cloud of airborne fibers, and the fibers are collected as a web on a mechanical conveyor and moved away from the fiberization apparatus. The fiber web is then consolidated, which may involve cross-lapping and / or longitudinal compression and / or vertical compression and / or winding around a mandrel to produce a cylindrical product for pipe insulation. Other consolidation processes may also be performed. A binder composition is conventionally applied to the fibers, preferably when they are an airborne cloud. Alternatively, it can be applied after collection on the mechanical conveyor, but this is less preferred. Conventional types of binder can be used with mineral wool fibers. After consolidation, the fiber web is passed to a curing device to cure the binder. Curing can be carried out at temperatures of 100 to 300 °C, such as 170 to 270 °C, 180 to 250 °C, or 190 to 230 °C. It is preferred that curing take place in a conventional curing oven for mineral wool production, where hot air is blown through the consolidated web, preferably operating at a temperature of 150 to 300 °C, such as 170 to 270 °C, such as 180 to 250 °C, or 190 to 230 °C. Curing can take place for a period of 30 seconds to 20 minutes, such as 1 to 15 minutes, or 2 to 10 minutes. Typically, curing is carried out at a temperature of 150 to 250 °C for a period of 30 seconds to 20 minutes. The curing process can begin immediately after the binder is applied to the fibers. Curing is defined as a process where the binder composition undergoes a physical and / or chemical reaction. In the case of a chemical reaction, this usually increases the molecular weight of the compounds in the binder composition, thereby increasing its viscosity, typically until the binder composition reaches a solid state. The cured binder composition then bonds the fibers together to form a structurally coherent fiber matrix. Alternatively, the curing of the binder in contact with the mineral fibers can take place in a heat press. Curing a binder in contact with the mineral fibers in a heat press has the particular advantage of allowing the production of high-density products. The invention will be described in further additional detail by reference to the drawing (Figure 1), which is a schematic representation of a section of a cupola furnace configuration that can be used to implement the invention. The plasma torch 1 comprises two separate tubular electrodes 2 and 3, to which direct current is supplied via conductors 4 and 5 from a voltage regulator unit, such as a rectifier and a thyristor 6 or a rectifier and an IGBT (insulated-gate bipolar transistor). Current is supplied to the thyristor 6 via a conductor 7 comprising a transformer 8. The tubular electrodes 2 and 3 are provided with cooling jackets, and cooling water is introduced into the cooling jackets by means of a pump 9 and recycled via a heat exchanger 10. A cooling medium is introduced into the heat exchanger 10 via a supply pipe 11 and discharged via an outlet pipe 12. The plasma torch 1 also comprises a pipe 13 for introducing carrier gas into a distribution chamber 14, from which it flows into the space between the electrodes 2 and 3. The plasma torch 1 is connected to the side wall of a cupola furnace 16 via a connecting pipe 15. The end of the connecting pipe 15 is inserted into a hole in the wall of the cupola furnace. In operation, the plasma jet 17, generated by the plasma torch 1, extends into the center of the cupola furnace and defines a cavity (dotted line 18) that is substantially devoid of solid material within the hot zone. The plasma jet 17 provides plasma heating to the hot zone. The cupola furnace also includes a waste inlet 19 located in the side wall of the furnace's hot zone, and in this case, above the orifice through which the plasma jet enters the cupola furnace. During operation, waste mineral material can be fed through the waste inlet 19 into the cavity 18 defined by the plasma jet. This can be achieved, for example, via a screw conveyor 20 rotatably mounted on a feed pipe 21 extending through the waste inlet 19, as shown in Figure 1. The plasma jet 17 melts the residual mineral material and the mineral material in the hot zone of the furnace, and the resulting mineral melt pools at the base of the cupola furnace. The mineral melt can then be removed from the furnace in the usual manner.

Claims

1. A process for preparing a mineral melt in a cupola furnace, wherein the mineral melt is for the formation of mineral wool, the cupola furnace comprising (i) a hot zone at the base of the furnace; (ii) a melt outlet in the hot zone; (iii) at least one plasma torch providing plasma heating to the hot zone by means of a plasma jet defining a cavity that is substantially devoid of solid material within the hot zone; and (iv) a waste inlet located in the side wall of the hot zone of the furnace and configured to supply waste mineral material to the cavity defined by the plasma jet, wherein the mineral material and the waste mineral material supplied to the furnace fuse to form the mineral melt. 2.The process as claimed in claim 1, wherein the amount of waste mineral material used is from 1 to 40%, preferably from 5 to 35%, more preferably from 10 to 20%, of the total weight of the waste mineral material and the mineral material.

3. The process as claimed in claim 1 or claim 2, wherein the waste inlet (I) is located in the furnace wall above the point where the plasma jet enters the furnace; and / or (II) has an area of ​​3 cm² to 320 cm², preferably from 20 cm² to 180 cm², more preferably from 50 cm² to 115 cm². 4.The process as claimed in any preceding claim, wherein the waste mineral material is introduced through the waste inlet by (a) a screw conveyor; or (b) pneumatic tube conveying, preferably wherein the pneumatic tube conveying uses recycled waste gas from the cupola furnace or gaseous nitrogen (N2) or any other gas that does not contain oxygen, preferably wherein the waste mineral material is introduced through the waste inlet by a screw conveyor. 5.The process as claimed in any preceding claim, wherein the cupola furnace comprises two to five plasma torches, preferably three or four plasma torches, located substantially in the horizontal plane around the perimeter of the furnace and substantially equidistant from each other, wherein the plasma jet from each plasma torch extends towards the center of the cupola furnace, and each plasma torch has an associated waste inlet.

6. The process as claimed in any preceding claim, wherein the waste mineral material is ground, preferably in a rod mill, before being introduced into the cupola furnace through the waste inlet.

7. The process as claimed in claim 6, wherein the waste mineral material is ground to a density of 500 kg / m³ to 1500 kg / m³, such as 700 kg / m³ to 1250 kg / m³, for example, 800 kg / m³ to 1000 kg / m³. 8.The process as claimed in any preceding claim, wherein (i) the furnace is equipped with at least one tuyere and / or an oxygen injection opening that provides a source of oxygen in the oxidation zone of the cupola furnace; (ii) the at least one plasma torch uses as a carrier gas nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2) or a mixture thereof, optionally wherein the enthalpy of the carrier gas is from 2.0 to 6.0 kWh / Nm3, such as from 3.0 to 5.0 kWh / Nm3, and preferably wherein the carrier gas is nitrogen (N2); (iii) more than 50% of the heating energy of the furnace is provided by the plasma torch; (iv) the temperature in the oxidation zone is less than 1400 °C; (v) the temperature in the hot zone is higher than the temperature in the oxidation zone; and / or (vi) water is substantially excluded from any area of ​​the furnace where the temperature is above 750 °C. 9.The process as claimed in any preceding claim, wherein (I) more than 60%, preferably more than 70%, more preferably more than 80%, even more preferably more than 90%, most preferably all of the cupola furnace heating energy is provided by at least one plasma torch; and / or (II) wherein the heating is provided in the hot zone solely by at least one plasma torch. 10.The process as claimed in any preceding claim, wherein (a) the temperature in the oxidation zone is from 600 °C to 1400 °C, preferably from 600 °C to 1300 °C, more preferably from 600 °C to 1200 °C, even more preferably from 600 °C to 1100 °C, especially from 600 °C to 1000 °C, most preferably from 600 °C to 900 °C, particularly from 600 °C to less than 850 °C; and / or (b) the temperature in the hot zone is above 800 °C, preferably above 900 °C, more preferably above 1000 °C, more preferably above 1100 °C, more preferably above 1200 °C, more preferably above 1300 °C, more preferably above 1400 °C.

11. The process as claimed in any preceding claim, wherein the mineral melt has the following composition expressed as oxides, in % by weight 12.The process as claimed in any preceding claim, wherein the proportion of Fe(2+) in the mineral melt is more than 80% based on the total Fe, preferably at least 90%, more preferably at least 95%, and most preferably at least 97% based on the total Fe.

13. The process as claimed in any preceding claim, wherein the carrier gas comprises, or consists of, at least one component of waste gas produced by the furnace, preferably wherein the at least one component of the waste gas is subjected to waste gas cleaning prior to its use as a carrier gas, more preferably the waste gas cleaning is to remove particles and / or water. 14.A process for manufacturing artificial vitreous fibers (MMVF) comprising the steps of (i) forming a mineral melt using a process as defined in any one of claims 1 to 13; (ii) fibrosing the mineral melt by an internal or external spinning process, preferably using a cascade spinneret; and (iii) collecting the formed fibers.

15. A cupola furnace for preparing a mineral melt according to a process as defined in any one of claims 1 to 13.

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