BACKGROUND TO THE INVENTION

This invention relates to a system and process for preheating raw material in the production of ferrochrome products. In particular, but not exclusively, the system and process are used in the production of ferrochrome products using a DC arc furnace.

The use of direct current (DC) arc furnaces in the production of ferrochrome is well-known. Raw material in the form of fines to chip fraction (submicron to 40mm sizes) chromite ores, reductants and fluxes are fed into the DC arc furnace where it is melted using electrical energy. Increasing electrical energy costs pose a threat to the financial viability of ferrochrome production processes. The use of waste energy arising out of the production process to preheat the raw material prior to feeding it into the smelting furnace offers an opportunity for the reduction in costs or the increase in yield, or a combination of both. However, capital requirements, technical complexity, control difficulties and similar challenges detract from the full benefit that could be expected of such preheating using known systems and processes.

Preheating is a known concept and is used in many mineral and pyro metallurgical processes. Preheating of furnace feed material as a specific processing step is also known. This preheating step generally includes the use of energy from combustible carbon monoxide (CO) present in furnace exhaust gasses to heat the raw material in a shaft kiln located above the furnace. An example of such shaft kiln is produced by Metso Outotec Oyj in which a porous raw material column is preheated before smelting it in a submerged arc furnace. Preheating is also used in solid reduction processes, which generally include partial pre-reduction of pelletised raw materials to a high level of reduction for feeding at high temperatures and smelting in a submerged arc furnace. An example of such a process is the Premus process developed by Xstrata PLC (now acquired by Glencore PLC). Another example of where pre-reduction of fine raw materials was used is in the applicant’s chrome direct reduction (CDR) process used at its Middelburg Ferrochrome facility. Yet another preheating process makes use of a fluidised bed preheater to preheat raw material prior to feeding it into a DC Arc Furnace.

A significant disadvantage of the known processes is that, in the preheating step, there is direct contact between the raw material and the hot gas used for heating it. Bringing the hot gas in direct contact with fine raw material has the risk that the fine material may be carried over into the gas stream, which then has to be separated and disposed of post preheating with the solids filtered and return to the feed stream. The direct contact between the raw material and the hot gas could also cause segregation of fine and coarse material, with associated poor gas flow and efficiency as a result. There is furthermore a risk of possible combustion (oxidation) of the carbonaceous materials contained in the raw material.

Another disadvantage of these known processes in which there is direct contact between the raw material and the hot gas is that a breakdown of the coarse and porous materials into fines aggregates forms bed blockages, which results in excessive pressure being required to force the gas through the bed. This, in turn, could cause channelling in the bed and, therefore, poor heat transfer. Yet another problem with bringing the raw material into direct contact with the hot gas is that it could cause softening of some of the raw material in the raw materials mixture, with associated agglomeration, sintering, clumping or similar negative effects in the physical nature of the raw material. These changes in the physical nature of the raw material cause subsequent handling problems downstream in the process.

These known processes also have limitations in the type of reducing agents that can be used due to the decomposition of the reducing agent into its component parts, some of which cause difficulties in up and downstream handling. These known processes have also been found to cause reoxidation of pre-reduced minerals and, accordingly, the subsequent loss of value. Furthermore, process variation in the smelting unit due to undetected variation in the feed material is also known to increase processing times, which adversely affect the smelting unit.

These known processes furthermore require high capital costs to erect processing plants and furthermore increase the component count which, in turn, results in higher operating costs and lower reliability of the plant. High off-gas volumes at elevated temperatures also require additional cooling and handling at the expense of electrical energy. It has further been found that the energy utilisation efficiency in these known preheating plants is less than ideal.

The abovementioned disadvantages associated with known preheating processes have to be weighed up against the possible benefit to be gained from the preheating of the raw material. There is accordingly a need for preheating of furnace feed raw material at lower capital requirements, improved simplicity and greater ease of control. It is accordingly an object of the invention to provide a system and/or process for preheating raw material in the production of ferrochrome products that will, at least partially, address the above disadvantages.

It is also an object of the invention to provide a system and/or process for preheating raw material in the production of ferrochrome products which will be a useful alternative to existing systems and/or processes.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention there is provided a process for preheating raw material in the production of ferrochrome products, the process including: extracting off-gas produced in a DC arc furnace; feeding the extracted off-gas to a hot gas generator; generating hot gas by using the off-gas as a fuel source in the hot gas generator; feeding the hot gas to at least one preheater unit; feeding the raw material to the at least one preheater unit; conveying the raw material and hot gas along discrete paths in the preheater unit such that there is no direct contact between the hot gas and raw material; heating the raw material in the preheater unit by means of conduction using the hot gas; and extracting heated raw material from the preheater unit for use in the DC arc furnace.

The step of heating the raw material may include using a counter flow heat exchanger. The process may include using a tube or plate type heat exchanger, preferably a shell and tube heat exchanger, and conveying the raw material in tubes of the heat exchanger while conveying the hot gas in an internal volume defined by a shell of the heat exchanger surrounding the tubes. The process may include securing the tubes in position by using parallel, spaced apart baffles in the internal volume and directing the flow of hot gas in the internal volume within the shell.

The process may include conveying the raw material through the heat exchanger under the force of gravity, preferably by arranging the tubes in a substantially vertical orientation.

The process may include feeding raw material to a buffer located at the top of the preheater unit and preferably above the heat exchanger.

The process may further include extracting heated raw material from a discharge hopper located at the bottom of the preheater unit and preferably below the heat exchanger.

The process may include monitoring and controlling the extraction of heated raw material from the preheater unit in response to operational requirements of the DC arc furnace. The process may further include automatically adjusting feeding parameters of the raw material into the preheater unit, preferably the buffer, based on extraction parameters of the heated raw material from the preheater unit.

The process may include monitoring and controlling hot gas properties in response to operational requirements of the DC arc furnace. This may include monitoring and controlling the temperature, humidity and chemical composition of the hot gas. The process may include introducing a secondary fuel source and/or secondary air flow into the hot gas generator.

The process may further include extracting dust and/or fumes from the preheater unit. This may include extracting dust and/or fumes from a feed hopper used in delivering raw material to the heat exchanger. The process may further include cleaning and discharging the dust and/or fumes extracted from the preheater unit in a cleaning sub-system, such as a separate, freestanding system for cleaning the extracted dust and/or fumes. The process may further include separating the particulates from the extracted stream of dust and/or fumes and feeding the particulates back into the preheater unit as part of the raw material stream.

In accordance with a second aspect of the invention there is provided a system for preheating raw material in the production of ferrochrome products, the system including: a hot gas generator for generating hot gas using off-gas produced in a DC arc furnace as a fuel source in the hot gas generator; at least one preheater unit housing a heat exchanger for heating the raw material using the hot gas produced by the hot gas generator; raw material feeding means for feeding raw material into the at least one preheater unit; extraction means for extracting heated raw material from the at least one preheater unit; and heated raw material feeding means for feeding the extracted, heated raw material to the DC arc furnace; wherein the heat exchanger is a counter flow heat exchanger defining discrete flow paths for the raw material and the hot gas respectively through the heat exchanger such that there is no direct contact between the raw material and the hot gas, thereby using conduction as the primary heat transfer mechanism.

The heat exchanger may be a tube or plate type heat exchanger, preferably a shell and tube heat exchanger. The heat exchanger may include tubes for conveying the raw material and an internal volume defined by the shell of the heat exchanger surrounding the tubes for conveying the hot gas around the tubes. Parallel, spaced apart baffles may be provided in the internal volume to secure the tubes in position and to direct the flow of hot gas within the internal volume in the shell.

The heat exchanger is preferably arranged such that the tubes are in a substantially vertical orientation so that the raw material is conveyed, in use, through the heat exchanger under the force of gravity. The at least one preheater unit may include a buffer in which the raw material is received. The buffer is preferably located at the top of the preheater unit and more preferably above the heat exchanger.

The at least one preheater unit may include a collection container, preferably a discharge hopper, in which heated raw material is collected prior to being discharged or extracted from the preheater unit. The discharge hopper may be located at the bottom of the preheater unit and preferably below the heat exchanger such that heated raw material is collected therein under the force of gravity as it exits the heat exchanger.

The hot gas generator may include a primary fuel source inlet through which the off-gas is fed into the hot gas generator, a secondary fuel source inlet through which a secondary fuel source is fed into the hot gas generator, a primary air inlet through which a primary air flow stream enters the hot gas generator and a secondary air inlet through which a secondary air flow stream enters the hot gas generator.

The system may include a control system for monitoring and controlling the operations of the system. The control system may include monitoring, measuring and/or control devices for monitoring, measuring and/or controlling the extraction of heated raw material from the preheater unit in response to operational requirements of the DC arc furnace, and/or the hot gas properties in ducting used to convey the hot gas from the hot gas generator to the preheater unit, the buffer and/or the discharge hopper. The system may include adjusting means for automatically adjusting feed parameters of the raw material into the preheater unit in response to extraction parameters of the heated raw material from the preheater unit. The adjusting means for automatically adjusting the feed and/or extraction parameters is preferably controlled by and/or form part of the control system. The system may further include adjusting means for automatically adjusting operational parameters of the hot gas generator to control the temperature, humidity and chemical composition of the hot gas. The adjusting means for automatically adjusting the hot gas operational parameters is preferably controlled by and/or form part of the control system.

The system may further include a dust and/or fumes extraction system for extracting dust and/or fumes from the preheater unit. The dust and/or fumes extraction system may be in fluid communication with a feed hopper of the preheater unit to extract dust and/or fumes from the feed hopper.

The system may further include a metering system for controlling the extraction of raw material from the at last one preheater unit. The metering system is preferably located below the at least one preheater unit and includes weighing means for weighing the raw material during extraction and feeding means for feeding the extracted raw material to the DC arc furnace. The metering system is preferably enclosed, airtight and insulated to counter heat losses and the ingress of air so as to mitigate the risk of combustion of the raw material.

The system may include a cleaning subsystem for cleaning and discharging the dust and/or fumes extracted from the preheater unit and a return line for feeding particulates separated from the extracted stream of dust and/or fumes back into the preheater unit as part of the raw material stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a schematic diagram of a system and process for preheating raw materials in the production of ferrochrome products in accordance with the invention; and

Figure 2 shows an enlarged detail view of one preheater unit of the system of Figure 1. DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms "mounted", "connected", "engaged" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings and are thus intended to include direct connections between two members without any other members interposed therebetween and indirect connections between members in which one or more other members are interposed therebetween. Further, "connected" and "engaged" are not restricted to physical or mechanical connections or couplings. Additionally, the words "lower", "upper", "upward", "down" and "downward" designate directions in the drawings to which reference is made. The terminology includes the words specifically mentioned above, derivatives thereof, and words or similar import. It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," and any singular use of any word, include plural referents unless expressly and unequivocally limited to one referent. As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items.

Referring to the drawings, in which like numerals indicate like features, a nonlimiting example of a system for preheating raw material in the production of ferrochrome products in accordance with the invention is generally indicated by reference numeral 10. The system 10 includes a direct current (DC) arc furnace in which raw material is melted using electrical energy. The furnace 12 is fed with a direct current electrical supply via an electrode column 14 forming part of an electrical circuit 16. The electrical circuit 16 is completed by an anode 18 located at the bottom of the furnace 12 (Figure 1 ) or as a separate anode device placed under or over the molten bath of the DC Arc furnace (not shown). The electrical circuit 16 includes a DC rectifier 20 supplied by a transformer 22 and an alternating current (AC) supply 24. The raw material is melted by an open DC arc between an end of the electrode 14 and the anode 18, and liquid alloy and slag are drawn off through custom tapping apertures or outlets 26 and 28 respectively. Alloy ingots 30 are typically cast from the liquid alloy drawn off through the outlet 26.

The off-gas produced in the smelting process is continuously extracted from the furnace 12 through a chute or duct 32. The off has is conveyed to a gas scrubbing plant 34 where it is cleaned for downstream use. Alternatively the off-gas may be stored for later use. The off-gas is at an elevated temperature and contains chemical energy in the form of, among other, carbon monoxide (CO) and hydrogen (H ₂ ). The off-gas is generally a mixture of CO, H ₂ , nitrogen (N), water vapour and other minor gasses as they occur in the furnace waste gas from time to time. From the scrubbing plant 34 the cleaned off-gas runs through a compressor 36 or suitable fan, which is used to get the off-gas to a desired pressure. The off-gas passes through a gas dryer 38 prior to being supplied to a hot gas generator 40. The hot has generator 40 is typically a combustion furnace or chamber. Although only one combustion chamber 40 is shown in Figure 1 , it is envisaged that multiple units or chambers could be used in alternative embodiments. The configuration and number of combustion chambers 40 will generally depend on the operational requirements of the system 10. It should therefore be clear that the combustion chamber 40 is suitably constructed for the temperature, composition and flow required for the process, and can be of a single unit or multiple unit configuration for proper and efficient production of hot gas for use in the pre-heating of the raw material. The hot gas generator 40 typically comprises the necessary fans, gas lines, gas control safety and control gear, instruments, ducts and vessel so constructed to withstand the desired process demands.

From the above description it should be clear that the fuel used in the hot gas generator 40 to produce the hot gas is the off-gas produced by the DC arc furnace 12 of the same system 10. A combustion air supply 42 is fed into the hot gas generator 40 for use in the combustion process.

The system 10 of the invention includes an optional secondary fuel supply 44 supplying fuel to the hot gas generator 40. The secondary fuel supply 44 could provide fuel in the form of a liquid fuel, liquid petroleum gas, methane gas or similar fuel. The secondary fuel is typically used during start-up of the hot gas generator 40 or to supplement the primary fuel supply, i.e. the supply of off-gas, as and when required. It is also envisaged that waste gas could be provided to the hot gas generator from a gas network on the ferrochrome production plant or facility. An optional secondary air supply 46 is also provided to supplement the air supply into the hot gas generator 40.

From the hot gas generator 40, the hot gas is supplied to at least one preheater. In the illustrated embodiment of Figure 1 , the hot gas is supplied to a heat exchange plant 48. The hot gas generator is preferably located in close proximity to the heat exchange plant 48 to minimise heat losses while conveying the hot gas. Insulated ducting is used to minimise heat losses while conveying the hot gas from the hot gas generator to 40 to the heat exchange plant 48.

The heat exchange plant 48 includes at least one and preferably a number of preheater units 50. Each preheater unit 50 includes a heat exchanger 52 for heating raw material using the hot gas. In the preferred embodiment of the system 10 the heat exchangers 52 are cross flow or counter flow heat exchangers. In particular, the heat exchangers 52 are of the tube or plate type heat exchangers. In the preferred embodiment of the system 10, the heat exchangers 52 are shell and tube heat exchangers in which the raw material is conveyed in tubes 52.1 of the heat exchangers and the hot gas is supplied into an internal volume 52.4 of the heat exchanger 52 surrounding the tubes, i.e. the volume around the tubes as defined by a shell 52.2 of the heat exchanger 52. Parallel, spaced apart baffles 52.3 extend from the shell 52.2 into the internal volume 52.4 and secure the tubes 52.1 in position while directing the flow of hot gas within the shell. The heat exchangers 52 are constructed to withstand the duties imposed on them by the abrasive nature of the raw material passing on one side and the hot gas passing on the other side of the heat exchanger elements, i.e. the tubes 52.1 in the illustrated embodiment.

The raw material is conveyed to the individual preheater units 50 where it is fed into the preheater unit using raw material feeding means. The raw material enters the unit through an inlet located at the top of the heat exchanger 52 into the area known as a feed hopper (not shown). The raw material can be fed to the feed hoppers as continuous streams or in batches. As shown in Figure 1 , the raw material is supplied from a raw material handling plant 54 where the raw material is prepared, particularly weighed off and blended, according to the requirements of the ferrochrome production process. The prepared raw material is conveyed to the at least one preheater unit 50, in particular the heat exchange plant 48, by way of conveyances 56. In the heat exchange plant 48 the raw material is accumulated in a buffer 58 located at the top of each preheater unit 50 from where it is fed into the heat exchanger 52. In use, the raw material moves along the tubes 52.1 of the heat exchanger 52, i.e. downward, under the force of gravity. The hot gas, in turn, enters the heat exchanger 52 through an inlet or port located vertically below the inlet of the raw material. In the preferred embodiment of the system 10 the hot gas inlets are located at the bottom of the heat exchanger 52. In this configuration, the hot gas and raw material move in substantially opposite directions through the heat exchanger 52. The hot gas inlet is arranged to provide effective flow patterns around the heat exchanger elements. Under natural conduction, the heat is extracted from the hot gas and flows via the heat exchanger element wall and into the raw material in contact with the other side of this heat exchanger element wall. The raw material in the heat exchanger tubes 52.1 is substantially a packed bed and, as such, is able to conduct heat on a particle-to-particle basis.

The hot gas is cooled down as it flows across the heat exchanger elements and heat is conducted to the raw material. The cooled hot gas is discharged through exhaust ducting 60 provided and arranged to dispose of the cooled gas stream safely.

Dust and/or fumes, produced incidental to the conveying, buffer stock holding and preheating of the raw material in the heat exchangers 52, are extracted through a custom designed extraction system 62 for cleaning and post treatment. The dust and/or fumes extraction system 62 is typically in fluid connection with the feed hopper. The dust and/or fumes extracted through the extraction system 62 is to be cleaned and discharged in a separate, freestanding system.

The raw material heated by the hot gas, i.e. the heated raw material, is extracted from the at least one preheater unit 50 using extraction means. The heated raw material is accumulated or collected in collection containers, preferably in discharge hoppers 64, located below the heat exchanger elements. The discharge hoppers 64 are illustrated as bottom discharge hoppers through which the heated raw material is discharged. As mentioned above, the raw material moves vertically down the preheater units 50 and accumulate in the discharge hoppers 64. In each preheater unit 50 the discharge hopper 64 forms a bottom section or floor of the preheater unit. The discharge hoppers 64 are insulated to reduce heat losses from the heated raw material. In use, the heated raw material is drawn from the discharge hopper 64 either as a continuous stream or in batches. As the heated raw material is discharged from the discharge hopper 64, the raw material resting above it moves downward under gravity to replace this loss of raw material from the preheater unit 50. At the same time, fresh raw material is charged to the feed hopper located at the top of the preheater unit 50. The raw material in the preheater units 50 is therefore automatically replenished through the discharge and charge process described above. In other words, the raw material is automatically fed into the preheater units 50 based on the discharge of raw material from the preheater units 50.

The system 10 includes adjusting means (not shown) for automatically adjusting feeding parameters of the raw material into the preheater unit 50 in response to extraction parameters of the heated raw material from the preheater unit 50. The feeding and extraction parameters could include parameters such as the weight, speed, rate, volumetric rate, temperature, composition, moisture content or any other relevant parameter of the raw material at the point of feeding and extracting raw material to and from the preheater unit 50 respectively. The adjusting means is typically controlled by the control system

Extraction of the heated raw material from the preheater units 50, in particular the discharge hoppers 64, is controlled according to the demands of the DC arc furnace 12. Sufficient volume of raw material at ambient temperature is maintained in the buffer 58 at the top of the heat exchanger 52 to provide continuous feed of material to the heat exchanger as the heated raw material is drawn off. A metering system 66 is located below the preheater units 50 for controlling the extraction of raw material from the preheater units. The metering system 66 includes suitable mechanical, electrical and control devices provided to ensure that the raw material is accurately extracted from the preheater units 50 according to the demands of the DC Arc furnace. The metering system 66 further includes weighing means for weighing the raw material during extraction and feeding means 68 for feeding the extracted raw material to the DC arc furnace 12. The feeding means 68 includes pipes or chutes 70 through which the heated raw material is conveyed to the DC arc furnace 12, which are arranged specifically for stability in the electrical circuit 16. In the system 10 of the invention the extraction, weighing and feeding of heated raw material occur in an enclosed and airtight arrangement, which is preferably insulated against heat losses and/or against the ingress of air so as to mitigate the risk of combustion of the raw material. In this description the words extracting and extraction, and their derivatives, should be interpreted broadly to describe any action of removing, taking out or discharging irrespective of whether such action is by force or not. For example, the heated raw material may be extracted from the preheater unit(s) (50) under external force or simply be allowed to exit from the preheater unit(s) freely, such as under the force of gravity only.

The system 10 of the invention includes a control system (not shown) for controlling operations. The objective of the control system is to execute the ferrochrome production process continuously while maintaining affective extraction of ferrochrome from the raw material provided. The control system typically forms part of a plant control system used in controlling operations of the ferrochrome plant. The control system includes sensors for sensing the raw material levels, including the discharge rates of the heated raw material from the preheater unit(s), and in response thereto feed fresh raw material into the feed hoppers. The temperature of the raw material is also continuously monitored as the raw material moves through the preheater units 50. The raw material temperatures are continuously communicated to the control system.

The control system includes field instrument devices, electronic signal collection devices, electronic processing units and associated equipment required for efficient control of the ferrochrome production plant as a whole or as subdivisions of the whole.

The control system includes monitoring and/or measuring devices installed in strategic locations throughout the system 10, including in the hot gas ducting used to convey hot gas from the hot gas generator 40 to the preheater units 50, the buffers 58 and discharge hoppers 64. The monitoring and/or measuring devices communicate the status information and control inputs to the control system to maintain operating parameters and safety limits. The temperature of the hot gas is continuously monitored and adjusted automatically according to process control parameters. These parameters may include operational parameters to ensure the optimal performance of the DC arc furnace and/or limits imposed by the process and equipment to ensure safe operation of the system 10. This includes using the control system to control operations of the hot gas generator 40 and the heat exchange plant 48 to provide hot gas at the desired temperature, humidity and chemical composition. For example, temperature setpoints are set via the control system and the volume of off-gas and the air-to-fuel ratio in the hot gas generator 40 are controlled in order to meet the requirements of the preheater units 50, and particularly the heat exchangers 52. As such, the control system not only controls the hot gas temperature in accordance with temperature setpoints and limits, but also controls the composition, in particular the amount of oxygen, in the hot gas stream. The operational parameters of the hot gas generator are automatically adjusted by adjusting means (not shown) in order to adjust hot gas properties such as the temperature, humidity and chemical composition of the hot gas.

The control system furthermore contains process control logic required for the safe start-up, shut down, emergency shut down and ramp up curves as required by the ducting, equipment or raw material used in the system 10.

An advantage of the system 10 in accordance with the invention is that there is no direct contact between the raw material and the hot gas being used to preheat the raw material. As a result there is no mixing of the raw material and hot gas. Heat is transferred in the heat exchanger(s) 52 from the hot gas to the raw material through conduction. As a result of the type of heat exchanger(s) 52 being used, the hot gas transfers heat by conduction to the heat exchanger elements which, in turn, transfer heat to the raw material descending on the opposite side of the heat exchange elements by conduction. It should be clear that the heat exchanger(s) 52 define discrete flow paths for the raw material and the hot gas through them respectively such that there is no direct contact between the raw material and the hot gas. In the system 10 of the invention, conduction is the primary heat exchange mechanism in the preheating of raw material.

From the above description it should be understood that the preheater units 50 provide buffer stock of raw material and preheats the raw material, to an elevated and controlled temperature, without direct contact between the raw material and the hot gas. As a result, the system 10 and the corresponding process for producing ferrochrome products in accordance with the invention address the disadvantages of the known systems and processes set out above. It is envisaged that, in the system 10, the improved electrical efficiency will result in increased production per unit electrical input, while avoiding at least some of the drawbacks of the known preheating techniques. It is believed that the system 10 and process in accordance with the invention have additional benefits, including a reduction in equipment count, reduced opportunity for oxidation in the feed material, avoidance of the consequences of oxidation in accurate temperature control and furnace feed control.

It will be appreciated that the above description only provides an example embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention. It is easily understood from the present application that the particular features of the present invention, as generally described and illustrated in the figures, can be arranged and designed according to a wide variety of different configurations. In this way, the description of the present invention and the related figures are not provided to limit the scope of the invention but simply represent selected embodiments.

The skilled person will understand that the technical characteristics of a given embodiment can in fact be combined with characteristics of another embodiment, unless otherwise expressed or it is evident that these characteristics are incompatible. Also, the technical characteristics described in a given embodiment can be isolated from the other characteristics of this embodiment unless otherwise expressed.