WO2017021895A1

WO2017021895A1 - Plant and method for vacuum degassing liquid steel

Info

Publication number: WO2017021895A1
Application number: PCT/IB2016/054667
Authority: WO
Inventors: Mauro MILOCCO; Federico MAZZARINI; Fabio TRIGATTI
Original assignee: Sms Meer S.P.A.
Priority date: 2015-08-06
Filing date: 2016-08-03
Publication date: 2017-02-09
Also published as: KR20180039647A; CA2994216A1; EP3332038B1; ES2742861T3; ITUB20152949A1; EP3332038A1; US20190002996A1

Abstract

The invention relates to a plant for vacuum degassing liquid steel, comprising: - at least one vacuum chamber 2, suitable to temporarily receive liquid steel inside it; - a vacuum generation system 10, connected to said at least one vacuum chamber 2 via an intake duct 20. The vacuum generation system 10 comprises at least two compression stages connected together in series, of which: a first compression stage 11 works closer to the aforesaid at least one vacuum chamber and is composed of one or more screw pumps 110; and a second compression stage 12 works farther with respect to said at least one vacuum chamber 2 to bring the gases at least to atmospheric pressure and is composed of one or more liquid ring pumps 120. The aforesaid one or more screw pumps are sized to be able to operate with compression ratios not exceeding 1:12 if the discharge pressure is atmospheric, and with compression ratios not exceeding 1:200 if the discharge pressure is comprised between 50 and 120 mbar absolute. Preferably, the screw pumps are sized to be able to operate with compression ratios comprised between 1:3 and 1:10 if the discharge pressure is atmospheric and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios of between 1:25 and 1:200, and preferably between 1:70 and 1:90. The present invention also relates to a method for vacuum degassing liquid steel.

Description

DESCRIPTION

"PLANT AND METHOD FOR VACUUM DEGASSING LIQUID STEEL"

Field of application

[0001] The present invention relates to a plant and method for vacuum degassing liquid steel.

[0002] The plant and the method according to the invention can be used for vacuum degassing with either the VD (Vacuum Degassing) technique or the VOD (Vacuum Oxygen Decarburisation). technique and for all applications where a vacuum treatment of liquid steel is required.

State of the art

[0003] The vacuum degassing process ( called for simplicity VD/VOD,. from the English "Vacuum Degassing " and "Vacuum Oxygen Decarburisation") respectively is a steel process that has as its main objective that of producing steels that meet high quality standards and stainless steels.

[0004] The vacuum treatment makes it possible to achieve extremely low levels of sulphur, hydrogen and nitrogen, improve the micro and macro purity of the steel, and, in the case of VOD, decarbonise (reduce. the carbon content) the steel.

[0005] Generally, VD/VOD systems are designed to operate round the clock and each treatment lasts from 35 to 120 minutes depending on the productivity required of the plant and on operating practices. [0006] Although various plant solutions exist aimed at meeting specific requirements (available installation space, required productivity) , normally a steel degassing plant consists of the following components, as shown in the general diagram in Figure 1:

[0007] -a vacuum chamber A, airtight to the outside, inside which the ladle L containing the liquid steel is housed.

[0008] -a vacuum generator B, i.e. a system able to aspirate gases until a pressure of less than 1 mbar absolute is achieved inside the vacuum chamber.

[0009] -an intake line C which places the vacuum Chamber A in communication with the vacuum generator B and the latter with the stack D through which the gases generated by the process are discharged;

[0010] -devices aimed at managing the process, installed along the intake line (valves illustrated below and gas pressure and temperature measuring instruments, a heat exchanger for cooling the process gas in output from the vacuum chamber) ;

[0011] -a dust separation unit F generally composed of a cyclone (to remove larger particles) and a filter (to retain smaller particles);

[0012] -a gas insufflation system G, usually argon, in some cases even nitrogen, for the agitation of the liquid steel and removal of impurities within it; [0013] -an insufflation system E^' of inert gas into the intake duct in order to manage the foamy slag, raising the pressure inside the isolated system;

[0014] -where the vacuum decarburisation of steel (VOD) is also provided for, an oxygen injector controlled by an auxiliary system is installed on the lid of the vacuum chamber .

[0015] The vacuum chamber A consists of a lid Al and a tank A2. Depending on which is the fixed part and which is the mobile part there are two types of construction: a "wheeled lid" when the tank is fixed and the lid mobile, and a "wheeled tank" in the opposite case.

[0016] Generally speaking, depending on the operating principle the vacuum generator B may be of two types: with a steam ejector/liquid ring (technical solution more popular in the past) or mechanical pumps (technology becoming more widespread recently) .

[0017] As shown in Figure 1, the devices for managing the process and installed along the intake line C usually comprise a valve VI to return the vacuum chamber to atmospheric pressure, a main valve V2 to isolate the vacuum chamber from the vacuum generator, a valve V3 for insufflating nitrogen to control the process.

[0018] The degassing plant in general is divided into two parts by the main valve V2. There are thus two volumes: a tank volume and a retained volume.

[0019] The tank volume is returned to atmospheric pressure after every vacuum treatment by opening the valve VI which effectively places the vacuum chamber in communication with the external environment. The retained volume, instead, is generally kept in a vacuum thanks to the main valve V2, which keeps it isolated from the external environment. The maintenance of the vacuum in the retained volume makes it possible to shorten the time required to lower the pressure in the ^'system by using it as a "plenum chamber" equalising the pressure between the tank and retained volume at the moment of opening the main valve V2. It should be noted that the tank is at atmospheric pressure before opening the main valve V2.

[0020] Generally, a vacuum degassing process comprises the following steps:

[0021] - positioning the ladle containing liquid steel inside the vacuum chamber and closing the lid;

[0022] - aspirating the gases contained inside the plant volume to achieve the required vacuum level (typically < 1 mbar) ;

[0023] - permanence at the operating pressure for the time deemed appropriate (typically from 15 to 25 minutes) to achieve the metallurgical objectives;

[0024] - restoration of the atmospheric pressure inside the vacuum chamber (opening of valve VI), and refining of the chemical analysis by additions of materials in precise quantities.

[0025] The intake gas is composed primarily of air up to a pressure of about 100-150 mbar, then of metal vapours, hydrogen and nitrogen coming from the steel. The suction capacity of the vacuum generation system automatically adjusts throughout the range of pressures. The operator is requested to perform an adjustment only in the case of abnormal chemical reactions inside the vacuum chamber (especially in cases of foaming of the slag, present in the ladle with the molten steel, to avoiding leakages of incandescent material from the ladle itself) .

[0026] The control of the entire process passes through the movement of the lid and/or tank and the command of the automatic cycles for the adjustment of the operating conditions of the system (i.e. of the working points of the vacuum generator in order to control the pressure inside the vacuum chamber) .

[0027] It is also known that during the entire degassing process a large amount of dust is produced.

[0028] The material constituting the dust derives mainly from the evaporation of metal elements present in the liquid bath, subsequently condensed along the intake line and the filter, from the reaction between the steel and the refractory and, to a lesser extent, from iron-alloys and scorifiers.

[0029] During a VD process approximately 0.1-0.2 kg of dust is produced per tonne of treated steel: during a complete treatment up to 20-40 kg may be produced (considering for example a ladle of a capacity of 200 tonnes of liquid steel) . A typical analysis of the dust composition reveals a significant content of Zn, MgO, CaO, Pb, Mn.

[0030] In the VOD process ("Vacuum Oxygen Decarburisation" , a vacuum process with insufflation of oxygen to achieve low levels of carbon in the liquid steel) the amount of dust generated may reach 800-1000 kg (for 200 tonnes of liquid steel) .

[0031] It is essential to have an effective dust collection system to preserve the vacuum generator from wear or clogging phenomena as well as to avoid dust emissions into the atmosphere.

[0032] If pressure filtration is required, a cyclone separator (tangential air intake) and a bag filter are installed in series on the intake line. However, filter installations also exist with an integrated cyclone.

[0033] Typically, the dust from these processes, because of its composition, burns very easily in the presence of oxygen. For this reason the bag filters (which are currently the most common technology for such applications) require frequent and efficient cleaning, which is typically done automatically after every treatment, by blowing inert gas (nitrogen) in counter- flow to the canvas bag, a technology known as "reverse pulse jet".

[0034] Aside from environmental requirements concerning atmospheric emissions, the need to install elements for dust abatement (bag filters and cyclone) or not, is determined by the degree of dust tolerated by the vacuum system to be installed.

[0035] To date, there are two vacuum generation technologies based on completely different operating principles: mechanical pumps and steam ejector systems. Vacuum generation with mechanical pumps

[0036] In the terminology commonly used in the steel industry, mechanical pump vacuum generation refers to a vacuum generator which provides for the installation in series of lobe type blowers (root pumps) and screw pumps (screw pump) as illustrated in Fig.2. In this case the screw pumps are also called "pre-vacuum pumps".

[0037] As a general principle, since each of these machines performs a compression of the aspirated gas, a compression "stage" is spoken of referring to one or more machines operating in the same pressure range between intake and discharge. [0038] The current most widespread plant solutions consist of a series of a screw pump and at least two root pumps in series, as shown in Figure 2.

[0039] The stages are conventionally named in ascending numerical order (stage 1, ..., stage n) starting from those closest to the vacuum chamber A. The last stage is that which finally discharges the gases into the atmosphere (pre-vacuum stage) . Each stage may consist of several pumps connected in parallel, as shown in Figure 3.

[0040] The criterion determining the arrangement in series is as follows: screw pumps are capable of operating with very high compression ratios (up to 1:1000) but with low volumetric flow rates; root pumps instead are able to dispose of large volumes of gas, but do not permit high compression ratios (typically about 1:6).

[0041] In typical VD/VOD installations, operatively, the screw pump alone is able to maintain a pressure of not less than 20-50 mbar inside the vacuum chamber, downloading the gases into the atmosphere. In order to achieve a higher degree of depression (< lmbar) the upstream installation of at least two stages of root pumps is required. The latter, thanks to the type of construction, (a double inner chamber alternately liberated and obstructed by the rotating lobes) are most effective in moving very rarefied gases, which gases at low pressures are.

[0042] In short, in stable operating conditions (i.e. disregarding the initial evacuation transient of the vacuum chamber starting from atmospheric pressure) , the early-stage root pumps aspirate the process gases at very low pressures (< lmbar) and deliver them to the screw pumps in the pressure range in which the latter operate with higher compression efficiency.

[0043] The main drawback of using mechanical pumps in a configuration as described above is related to the need to perform filtration of the aspirated gases in order to retain the solid particulate which could block and/or damage the rotating mechanical bodies (seizure) and possibly contaminate the lubrication oil (contained in the gear chamber in the case of deterioration of the gaskets) . The root pumps - while not meant for use in a pulverulent environment - would theoretically be capable of treating pulverulent gases without running into operational problems of seizure. In the long run however oil contamination problems would arise. The biggest problem relates to the screw pumps which would be forced - without filtration - to treat the pulverulent gases discharged by the root pumps, incurring in the aforesaid seizure problems and leading to immediate blocking of the system.

[0044] In describing the typical operating conditions of a vacuum generation system with mechanical pumps reference is generally made to 4 automatic cycles which determine the functioning of the main devices installed (valves, filter, pumps ) :

[0045] - activation cycle of the system: the pumps are started and the volume until the main valve ("retained volume") is evacuated reaching a final pressure typically < 5 mbar; the vacuum chamber at this stage remains at atmospheric pressure and the pumps are kept at a minimum rotation speed;

[0046]- Degassing cycle: the main valve opens to equalise pressure in a controlled manner between the vacuum chamber and the "retained" volume; the slow equalisation is designed not to overstress the system from a mechanical point of view (pumps and filterbags) and avoid the instantaneous and violent oxidation of the pyrophoric dust remaining on the surface of the filter bags after the previous treatments. The pumps gradually speed up to reach the maximum rotation speed. During the lowering of the pressure the vacuum level in the system can be controlled by slowing down/by-passing the pumps or by. insufflating nitrogen. Typically the process pressure (< lmbar) is reached in- 6-8 minutes. [0047] - Stop vacuum cycle: when degassing is complete, the main valve closes and the vacuum chamber is returned to atmospheric pressure (permitting the subsequent opening of the lid and addition of materials) .

[0048] - Cleaning cycle: with the pumps isolated, the filter bags are cleaned by means of a system of nitrogen blows, the cleaning cycle being then followed by a dust discharge cycle as necessary.

[0049] Once the cleaning cycle of the bags is compete, the retained volume is again evacuated (up to pressures < 5mbar) preparing the system for the next degassing cycle.

[0050] The cleaning of the filter bags is a crucial aspect for the performance of the mechanical pump system because :

[0051] -an excessive accumulation of dust on the bags increases the pressure losses through the filter, limiting the minimum pressure which can be reached inside the vacuum chamber;

[0052] -possible damage of the bags causes large quantities of dust to reach the pumps. The operation of the system may thus be jeopardised if the cleaning and maintenance of filters is not properly conducted (correct setting of the wash cycle with nitrogen, regular inspections of the bags ... ) .

Vacuum generation with ejector pumps [0053] Ej ector vacuum generators use as a propellent fluid the superheated steam generated in a boiler or coming from other sources. As a result of the acceleration of the steam and the architecture of the ejector, the process gas is aspirated and compressed.

[0054] Each ejector is sized to compress a given quantity of gas, achieving a specific ratio between the intake and discharge pressure (typically to the order of 1:5/1:15). To operate between the pressure required by the process (1· mbar) and atmospheric pressure (1000 mbar) several different ejectors operating in series are therefore required.

[0055] In this case too, in the arrangement in series, each ejector is considered as a compression "stage". A stage may however be composed of several ejectors in parallel to increase the suction capacity of the system at higher pressures (typically required during the evacuation phase of the vacuum chamber) .

[0056] Figure 4 shows a plant layout of a typical ejector pumping station where SI, S2, S3 and S4 indicate ejector stages, CI, C2 and C3 inter-stage condensers and P a collection tank or "hot pit". The S3 and S4 stages, in this particular case, consist of pairs of A/B ejectors operating in parallel. The activation sequence of the individual stages is usually controlled by the pressure reached by the vacuum chamber, and is as follows (with reference to Fig. 4) : S4-S3-S2-S1.

[0057] To ensure maximum efficiency of the ejector system (disposal of the maximum flow of process gas), heat exchangers are installed in series with the ejectors to condense the steam contained in the main gas flow.

[0058] The steam, in fact, acts only as a propeller - to aspirate the process gases and condense as the pressure increases and the temperature decreases .

[0059] The steam is thus made to condense inside the "condensers" which are drained into a tank, called a "hot pit" .

[0060] It is clear that, in the absence of filtering systems upstream of the ejector groups, the condensed water has a higher concentration of dust, thus requiring appropriate wastewater treatment plants and maintenance operations for the disposal of the sludge channelled into the "hot pit".

[0061] Fig. 4 shows a diagram of a typical ejector consisting of four compression stages.

[0062] A variation of this diagram provides that the fourth stage, or alternatively a possible fifth stage, consists of a liquid ring pump in place of an ejector. This solution is generally preferred in systems with limited steam availability or where required by plant or process requirements (limited space for installation, need to operate stably at pressures above 100 mbar for VOD systems) .

[0063] The liquid ring pump is a mechanical, centrifugal- type pump in which the compression of the gas, by means of its confinement in a variable (gradually reduced) volume, is consequential to the rotation of a liquid ring generated by a centrifugal effect of a rotor, eccentric to the casing (body) of said pump.

[0064] With the exception of the composition of the pumping system and of the dust abatement group connected thereto, the operation of an ejector/liquid ring plant passes through operational sequences entirely similar to those described for the mechanical pumps.

[0065] For the ejector systems it is not necessary, for the purpose of protecting the pumping system, ■ to abate the dust to the extent of requiring the installation of a bag filter since it lacks the geometric tolerances required by the mechanical system, typical of root or screw pumps.

[0066] On the other hand, in some systems, to minimise maintenance (cleaning of the ejectors and hot pit water treatment) a cyclone or even a bag filter with related automatic cleaning system may be installed.

[0067] Lastly, it is to be noted that in. the absence of filter elements, a large amount of dust is retained by the injected steam and by the water of possible liquid ring pumps. The condensed steam between one ejector stage and another helps to retain some of the dust generated during the process. The condensed water is drained, as mentioned above, into the "hot pit" (indicated as P in Fig. 4) . Also the possible liquid ring in its contact with the process gas helps to retain part of the residual dust. It follows that in a liquid ring/ ejector system the amount of dust contained in the gases discharged to the stack is very low.

[0068] In conclusion, the main difference as regards the system layout between ejector systems and mechanical pump systems lies in the presence of a bag filter (with all the auxiliary elements for cleaning the bags and discharging the dust) , required in the latter case to preserve the integrity of the machines.

[0069] The main limitation of injector vacuum generation systems lies in their complexity and high plant and running costs.

Presentation of the invention

[0070] Consequently, the purpose of the present invention is to eliminate entirely or in part the drawbacks of the prior art mentioned above, by providing a plant and method for vacuum degassing liquid steel combining the engineering/operational simplicity of a mechanical pump plant with the possibility to operate without filter systems of an ejector plant.

[0071] A further purpose of the present invention is to make available a plant for vacuum degassing liquid steel which is operatively more reliable.

[0072] A further purpose of the present invention is to make available a plant for .vacuum degassing liquid .steel which is cheaper to run.

[0073] A further purpose of the present invention is to make available a plant for vacuum degassing liquid steel which is at least comparable to conventional systems with mechanical pumps, in terms of plant costs.

Brief description of the drawings

[0074] The technical characteristics of the invention, according to the aforesaid purposes, can be seen clearly from the contents of the following claims and the advantages of the same will be more clearly comprehensible from the detailed description below, made with reference to the appended drawings, showing one or more embodiments by way of non-limiting examples, wherein:

[0075] -Figure 1 shows a general diagram of a steel degassing plant;

[0076] -Figure 2 shows a general diagram of a conventional vacuum generation system with mechanical pumps of the root and screw type;

[0077] -Figure 3 shows a general diagram of a conventional vacuum generation system with mechanical pumps of the root and screw type, with each stage composed of several pumps in parallel;

[0078] -Figure 4 is a diagram of a conventional ejector vacuum generation system;

[0079] -Figure 5 shows a general diagram of a liquid steel vacuum degassing plant according to a preferred embodiment of the present invention;

[0080] -Figure 6 shows a general diagram of a liquid steel vacuum degassing plant according to an alternative embodiment of the present invention;

[0081] -Figure 7 shows a general diagram of the vacuum generation system in a liquid steel vacuum degassing plant according to a preferred embodiment of the present invention; and

[0082] -Figure 8 shows a general diagram of a liquid ring pump .

Detailed description-

[0083] With reference to the appended drawings reference numeral 1 globally denotes a plant for vacuum degassing liquid steel according to the invention.

[0084] The plant 1 according to the invention can be used for vacuum degassing with either the VD (Vacuum Degassing) technique or the VOD (Vacuum Oxygen

Decarburisation) technique and for all applications where a vacuum treatment of liquid steel is required.

[0085] Here and henceforth in the description and the claims, reference will be made to the vacuum degassing plant of liquid steel 1 in conditions of use.

[0086] According to a general embodiment of the invention, the plant for vacuum degassing liquid steel comprises:

[0087] - at least one vacuum chamber 2, suitable to temporarily receive liquid steel inside it; and

[0088] - a vacuum generation system 10, connected to the aforesaid at least one vacuum chamber 2 via an intake duct 20.

[0089] The vacuum chamber 2 may be of any type suitable for the purpose.

[0090] Preferably, the vacuum chamber 2 is configured so that the liquid steel is brought inside via a ladle L, but it may also be used directly to receive the liquid steel .

[0091] In the first case, as shown in Figures 5 and 6, the vacuum chamber 2 comprises a tank 3, which defines the volume of the chamber 2 and is suitable to receive therein the ladle L, and a lid 4 suitable to seal the tank 3 tight when the ladle L is housed therein. The vacuum chamber may be of the "wheeled lid" type when the tank is fixed and the lid mobile or of the "wheeled tank" type in the opposite case.

[0092] Advantageously, as shown in Figures 5 and 6, the vacuum chamber may be fitted with an insufflation system 30 of a washing gas, in. some cases even nitrogen, for the agitation of the liquid steel and removal of impurities within it. In particular, this insufflation system 30 is designed to feed one or more porous septums located on the bottom of the ladle.

[0093] In the second case, according to an embodiment not illustrated in the appended Figures, the vacuum chamber 2 may be configured to directly house within it the liquid steel acording to an RH process. In this case, the liquid steel is transferred temporarily from the ladle inside the chamber. To such purpose the vacuum chamber is connected to a ladle via two ducts: a delivery duct through which the molten steel from the ladle is driven by the difference in pressure inside the vacuum chamber, and a return duct, through which the treated molten steel flows back from the vacuum chamber inside the ladle.

[0094] According to the invention, as shown in Figures 5 and 6, the vacuum generation system 10 comprises at least two compression stages connected to each other in series, of which:

[0095] -a first compression stage 11 works closer to the aforesaid at least one vacuum chamber 2 and consists of one or more screw pumps 110; and

[0096] -a second compression stage 12 works farther away from the aforesaid at least one vacuum chamber 2 to bring the gas at least to atmospheric pressure and consists of one or more liquid ring pumps 120.

[0097] The aforesaid one or more screw pumps 110 are sized to be able to operate with compression ratios not exceeding 1:12 if the discharge pressure is atmospheric, and with compression ratios not exceeding 1:200 if the discharge pressure is comprised between 50 and 120 mbar absolute.

[0098] As will be specified below, the aforesaid one or more screw pumps 110 are thus sized in a radically different manner to conventional screw pumps.

[0099] Preferably the aforesaid one or more screw pumps 110 are sized to be able to operate with compression ratios comprised between 1:3 and 1:10 if the discharge pressure is atmospheric and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios of between 1:25 and 1:200, and preferably between 1:70 and 1: 90.

[00100] Thanks to the fact of operating in the aforesaid compression ratio ranges, the aforesaid one or more screw pumps 110 are sized imposing internal tolerances (rotor/rotor and rotor/case) , much higher than those provided for in the conventional screw pumps used as pre-vacuum stages as described above. This way, the aforesaid screw pumps 110 are able to operate in direct contact with pulverulent gases with high concentrations of dust without contraindications for the moving mechanical parts and thus without incurring in the typical problems of screw pumps used as pre-vacuum stages in conventional, mechanical degassing systems.

[00101] This is made possible by the fact that according to the invention the screw pumps are used at stages closer to the vacuum chamber and by the choice to operate such pumps in the aforesaid compression ranges.

[00102] Operationally, the work of compressing the gas is completed by the aforesaid one or more liquid ring pumps which define the second compression stage (final), further away from the vacuum chamber, exploiting, the fact that the liquid ring pumps are insensitive to dust.

[00103] Advantageously, the liquid ring pumps also perform an important function of retaining the solid particles dragged along by the main flow of the gases. The pump service water is therefore used to trap the dust generated by the degassing process and then collect it in a single point. This way the dust emission at a possible discharge stack 40 is minimised, ensuring low environmental impact.

[00104] Thanks to the invention the vacuum generation system 10 is thus able to aspirate directly from the aforesaid at least one vacuum chamber 2 gases containing dust · in high concentrations, without the contraindications typical of a conventional mechanical pump system.

[00105] Conventionally, contrary to the provisions of the present invention, screw pumps are instead used in the vacuum generation systems of degassing plants to define the compression stages furthest away from the vacuum chamber A. These pumps (pre-vacuum) , despite operating with compression ratios between 1:1 and 1:50, and preferably between 1:2 and 1:40, are designed to work with compression ratios up to 1:1000 with discharge at atmospheric pressure. Conversely, the screw pumps 110 according to the invention are sized to operate at maximum compression ratios of 1:12 with discharge at atmospheric pressure. The traditional screw pumps must therefore be built with very strict internal tolerances (rotor/rotor and rotor/case) . This makes them particularly sensitive to the presence of dust in the gases treated.

[00106] Thanks to the present invention, it is therefore possible on the one hand to liberate the design of a liquid steel degassing plant from the installation of a filtration device (usually a bag filter) required in the case of mechanical pumps, and on the other to drastically reduce the plant costs entailed by a conventional steam ejector system.

[00107] Advantageously, the vacuum generation system 10 is sized to bring the vacuum chamber 2 to a degree of vacuum between 0.2 and 5 mbar, and preferably between 0.5 and 1.-5 mbar. As a result, the vacuum generation system 10 is sized to generate total compression ratios between 1:5,000 and 1:200.

[00108] As regards the sizing of the vacuum generation system 10 according to the present invention, the possible combinations in terms of number of screw pumps 110 and liquid ring pumps 120 are dictated by the design choices from time to time made so as to minimise the number of machines installed to get the level of performance required by the process, i.e. evacuation times of the vacuum chamber limited and degree of final vacuum approximately < 1 mbar.

[00109] Advantageously, the vacuum generation system 10 may comprise one or more intermediate compression stages, positioned in series between the first stage 11 and the second stage 12 and each composed of one or more screw pumps 110 having similar characteristics to those of the first stage 11.

[00110] The term "similar characteristics" is taken to mean that said one or more screw pumps of the intermediate stages are sized to operate in the same compression ranges as the screw pumps of the first stages, thus making it possible to adopt internal tolerances (rotor/rotor and rotor/case) much higher than those provided for in conventional screw pumps. The size of the screw pumps of the intermediate stages may be the same or different to that of the screw pumps of the first stages. The choice of size is dictated by the sizing of the vacuum generation system.

[00111] One or more of the aforesaid compression stages

(first, second or intermediate) may each consist of two or more pumps connected in parallel.

[00112] According to embodiments not shown in the appended figures, the vacuum generation system may consist of two or more parallel pumping modules, each of which is composed at least of a first compression stage 11 with screw pumps and a second compression stage 12 with liquid ring pumps.

[00113] The total number of pumps installed per module and the number of modules is defined in the design phase with the objective of optimising the installation and minimising the consumption of auxiliary elements (water, nitrogen, electricity) .

[00114] Advantageously, modular configurations may be adopted for the vacuum generation system 10, i.e. separable into units installed in parallel, or "hybrid" installations where the pumps are grouped on two stages without modularity.

[00115] Preferably, the vacuum generation system 10 can be isolated from the rest of the system by closing appropriate isolation valves installed immediately upstream of the pumps.

[00116] Preferably, as shown in Figures 5 and 6, the intake duct 20 comprises a by-pass duct 21 able to exclude from the gas flow the compressor stages formed of the screw pumps 110. This solution can be adopted both in the case of a modular structure, and a non-modular structure.

[00117] Operatively, as will be resumed below, the presence of the aforesaid by-pass 21 may be used to exclude the screw pumps from functioning in some stages of the degassing process.

[00118] Preferably, each of the screw pumps 110 used in the degassing plant 1 according to the invention comprises two screw rotors, kinematically synchronised with each other via an electric axis.

[00119] For the connection and synchronization of the two screw rotors these pumps do not use the conventional "mechanical axis", where an engine transmits movement to a screw rotor while the other rotor is dragged/ synchronised by means of a series of gears in oil bath.

[00120] The term "electric axis" means the software synchronisation of a pair of engines by means of an inverter (one for each screw) and a pair of encoders. The software instantly manages the parameters of the two inverters so that the rotors are constantly synchronised. Furthermore the two encoders control the angular deviation of the axes of the screw rotors, so that these are perfectly parallel to each other.

[00121] Operatively, any functional anomaly (e.g. internal friction due to dust build-up) results in an increase in torque and current absorption of the motors (or of one of them) and, consequently, a possible deviation in the angular speed of the rotors. Advantageously, the software can act on the speed in real time until equilibrium is restored, avoiding stresses and overheating of the pump.

[00122] Compared to a solution with a mechanical axis, this electric axis configuration does not require oil for the lubrication of the gears. The absence of lubrication oil is an advantage. In fact, due to the possible difference in pressure between the compression chamber (lower pressure) and possible (concurrent) gear chambers (higher pressure) , the oil can be aspirated into the process . gas, mixing with the dust and generating obstructions. Similarly, in certain operating phases, dusty gases can inundate the gear chambers polluting the oil .

[00123] The liquid ring pumps 120 used in the degassing plant 1 according to the present invention are of the type known per se and their operation is therefore well known to a technician of the sector. A detailed description of the same is therefore not provided but merely reference to a number of concepts useful for introducing some particular elements.

[00124] In particular, the liquid ring pumps used in the present invention may have the structure shown in Figure 8.

[00125] As shown in Figure 8, a liquid ring pump compresses the process gas G' between an eccentric vane rotor 121 and a ring 122 of water, called service water W. Operatively, the dust carried by the process gas G¹ necessarily comes into contact with the service water W which acts as a collector. The pump 120 ejects the compressed gas G" together with a minimum amount of pulverulent service water. The mixture G " + W of gas and pulverulent water reaches a separator 123 which separates the gas (now at atmospheric pressure and directed to the stack) from the "dirty "water which is collected in the lower part of the separator 123. Advantageously, a replenishment 124 of. the water W is provided to offset the losses from evaporation.

[00126] More specifically, the service water W can be handled in two ways: in an open circuit or closed circuit .

[00127] With closed-circuit management the water W is recirculated until the saturation limit of dust, at which the pump performance drops. At this^" point all the service water W is discharged and replaced with clean water.

[00128] With open circuit management, the water is continuously discharged from the separator (through the opening 125 illustrated in Figure 8), while a line of clean water 124 continuously tops up the service circuit of the liquid ring pump.

[00129] Advantageously, the resulting water contains dust which is now inert and can be handled in two different ways.

[00130] According to a first method, the pulverulent water is collected in a decanting bath with an overflow which leads to a second bath. From here the pulverulent water is sent on to a water treatment plant, by -means of centrifugal pumps, and treated therein in the conventional way.

[00131] According to a second method, as shown schematically in Figure 7, the pulverulent water leaving the separator can be filtered on site using known methods .

[00132] Advantageously, as shown in Figure 7, the plant

1 comprises an auxiliary unit 50, which, in addition to replenishing the water dispersed by the liquid ring pumps in the process gases, separates the dust contained in the water and recirculates it to the pump.

[00133] A continuous cycle operation guarantees both the controlled removal of the dust (avoiding internal build-up) and optimal operation of the liquid ring pump 120 thanks to the cooling and cleaning of the top-up water .

[00134] The auxiliary unit 50 may be centralised or located on board of each liquid ring pump or module, maintaining however the same functions.

[00135] Alternatively to the aforesaid auxiliary unit

50, the plant 1 may comprise at least one continuous replacement device of the service water used by the liquid ring pump, without recirculation, with non- returnable water.

[00136] Advantageously, as shown in Figures 5 and 6, in the section comprised between the vacuum chamber 2 and the vacuum generation system 10 the intake duct 20 comprises^' a connection branch 28 to the atmosphere equipped with a first control valve 23. This first control valve 23 is opened at the end of the degassing process to return the vacuum chamber 2 to atmospheric pressure before taking out the treated liquid steel.

[00137] Advantageously, as shown in Figures 5 and 6, in the section comprised between the vacuum chamber 2 and the vacuum generation system 10 the intake duct 20 may comprise a connection branch 29 to a tank (not shown) containing inert gas (nitrogen or argon) , equipped with a second control valve 24. The inert gas can be insufflated by opening the second valve 24 in order to manage the foamy slag, raising the internal pressure.

[00138] According to a preferred embodiment illustrated in Figure 5, the degassing plant 1 does not comprise a filtration device of the gases, which leave the vacuum chamber 2 and have to pass through the vacuum generation system 10. Regardless of the concentration level of the dust in said gases, the gases in output from the vacuum chamber 2 are aspirated directly by the vacuum generation system without a preventive gas filtration step. As noted previously, this is possible thanks to the present invention .

[00139] According to an alternative embodiment illustrated in Figure 6 the degassing plant 1 may comprise at least one filtration device 25 of the gases leaving the vacuum chamber 2 and passing through the vacuum generation system 10. Such a filtration device 25 is arranged between the vacuum chamber 2 and the vacuum generation system 10.

[00140] Operatively, the gases exiting the vacuum chamber 2, before being aspirated by the vacuum generation system, are subjected to filtration in order to abate at least partially the dust content present in the gases. Thanks to the present invention, the abatement of the dust may be partial and bland, given that the possible presence of dust does not affect the operation of the vacuum generation system 10. The preventive filtration step may be provided so as to optimise the management of dust in the system, reducing the load of dust to be managed by means of the liquid ring pumps.

[00141] The aforesaid filtration device 25 may consist of a bag filter, a cyclone or of an integrated bag filter and cyclone system.

[00142] In particular, according to the alternative embodiment illustrated in Figure 6, the plant 1 comprises at least an isolation valve 22 which is installed on the intake duct 20 between the vacuum . chamber 2 and the filtration device 25. Such isolation valve 22 is placed downstream of the branching point of the intake duct 20 into the aforesaid connection branch 28 to the atmosphere. The isolation valve 22 divides the plant 1 into two parts, thus identifying two volumes. A first part comprises the vacuum chamber (tank volume) ; the second part comprises the filtration device and the vacuum generation system (retained volume) .

[00143] Operatively, the tank volume is returned to atmospheric pressure after every vacuum treatment by opening the aforesaid first control valve 23 which places the vacuum chamber in communication with the external environment. The retained volume may, instead, be always kept in a vacuum thanks to the isolation valve 22 which effectively keeps it airtight. The maintenance of the vacuum of the retained volume makes it possible to shorten the time required to lower the pressure in the system by using it as a "plenum chamber" equalising the pressure between the tank and retained volume at the moment of opening the isolation valve 22.

[00144] The presence of the isolation valve 22 is preferred in the case in which the plant 1 is equipped with a filtration device 25 (in particular if it is a bag filter) as shown in Figure 6. In this case, the retained volume is very high due to the presence of the filtration device. [00145] Advantageously, in the case in which the plant

1 is not equipped with a filtration device 25 (see Figure 5), the isolation valve 22 need not be installed, since, in the absence of the filtration device,, the retained volume is reduced and therefore the advantages associated with maintaining said volume in a vacuum are limited.

[00146] Advantageously, in the case in which the plant

1 is used for the vacuum degassing with VOD (Vacuum Oxygen Decarburisation) technique it may comprise a heat exchanger (not shown in the appended figures) for cooling the process gases. In fact, with the VOD technique, as a result of the injection of oxygen and consequent decarburisation of the steel, the temperatures involved increase significantly. The heat exchanger should be placed upstream of the possible filtration device 22 and downstream of the possible isolation valve (if present), in the second part of the system (retained volume) .

* * *

[00147] The present invention also relates to a method for vacuum degassing liquid steel.

[00148] In particular, the method according to the invention may be implemented in a degassing system according to the invention, in particular as described above. The parts in common with the plant 1 described above have been indicated using the same alpha-numerical references.

[00149] According to a general embodiment of the invention, the method for vacuum degassing liquid steel comprises the following operating steps:

[00150] a) providing at least one vacuum chamber 2 suitable to temporarily receive liquid steel inside it;

[00151] b) placing liquid steel in said vacuum chamber

2;

[00152] c) evacuating the vacuum chamber 2 through a vacuum generation system 10 creating in said chamber a predefined degree of vacuum and maintaining it for a predetermined period of time so as to complete the operation of degassing the liquid steel; and

[00153] d) bringing again the vacuum chamber 2 to atmospheric pressure and withdrawing the degassed liquid steel .

[00154] According to the invention, the vacuum evacuation step c) is conducted by means of a vacuum generation system 10 comprising at least two compression stages connected together in series, of which:

[00155] -a first compression stage 11 works closer to the aforesaid at least one vacuum chamber 2 and consists of one or more screw pumps 110; and

[00156] -a second compression stage 12 works farther away from the aforesaid at least one vacuum chamber 2 to bring the gas at least to atmospheric pressure and consists of one or more liquid ring pumps 120.

[00157] The aforesaid one or more screw pumps 110 are sized to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1:200, if the discharge pressure is comprised between 50 and 120 mbar absolute..

[00158] Preferably, the aforesaid one or more screw pumps . 110 are sized to be able to operate with compression ratios comprised between 1:3 and 1:10 if the discharge pressure is atmospheric, and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios of between 1:25 and 1:200, and preferably between 1:70 and 1:90.

[00159] Preferably, in the aforesaid evacuation step c) , the vacuum chamber 2 is brought to work at a degree of vacuum between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar.

[00160] According to a preferred embodiment of the method, the evacuation step c) provides for the direct aspiration of the gases from the vacuum chamber 2 through the aforesaid vacuum generation system 10 without a preventive filtration step of the gases, independently of the level of dust concentration in the gases themselves. [00161] According to an alternative embodiment of the method, the evacuation step c) provides for the aspiration of the gases from the vacuum chamber 2 through the aforesaid vacuum -generation system 10 with a preventive filtration step of the gases, to reduce the dust concentration in said gases before their passage through the vacuum generation system 10.

[00162] Preferably, the evacuation step c) comprises:

[00163] - an initial evacuation step cl) wherein the vacuum chamber 2 is brought from atmospheric pressure up to about 300 mbar using only the liquid ring pumps of the vacuum generation system 10; and

[00164] - a final evacuation step c2 ) , wherein the vacuum chamber 2 is brought from the pressure of about 300 mbar to the predefined degree of vacuum, also using the screw pumps.

[00165] This operating mode makes it possible to minimise the amount of dust which the screw pumps must handle, to the benefit of the operation of such pumps. This operating mode takes advantage of the presence of the by-pass 21 which is present on the intake duct and which permits the exclusion of the screw pumps from the passage of the gases.

[00166] Advantageously, during the evacuation step c) the suction capacity of the vacuum generation system 10 can be varied to reduce any phenomena of foaming of the slag in the liquid steel. The suction capacity is varied by^■ slowing or excluding one or more of the pumps of the vacuum generation system 10, preferably the liquid ring pumps 120.

[00167] Preferably, the above change in suction capacity is carried out when the internal pressure of the vacuum chamber 2 is between 300 mbar and 1 mbar, i.e. during the final evacuation step c2) .

[00168] Preferably, the method according to the invention comprises a ^' step f ) of treating the service water used by the aforesaid one or more liquid ring pumps 120. This treatment step f) is carried out preferably during the evacuation step c) . The treatment consists of filtering the dust from the water or of continuously replacing the water.

[00169] Advantageously, the method comprises a step e) of mixing the molten steel at least during the evacuation step c) , in particular by insufflating inert gases into said steel.

[00170] The invention permits numerous advantages to be achieved, in part already described.

[00171] The plant 1 for vacuum degassing liquid steel according to the invention combines the plant simplicity of a mechanical pump plant with the possibility to operate without the filter systems of an ejector plant.

[00172] Thanks to the invention it , is therefore possible to minimise the equipment installed in a degassing plant .

[00173] This way it ensures greater flexibility in the design phase (layout and auxiliaries) as well as allowing operation of the system while minimising^' maintenance costs and possible repairs of the parts most subject to wear in a conventional system. In particular, it reduces periodic inspections and eliminates the need to replace the filter bags.

[00174] The plant 1 for vacuum degassing liquid steel according to the invention is therefore:

[00175] -operatively more reliable; and

[00176] -cheaper to run.

[00177] In terms of plant costs, the plant 1 for vacuum degassing liquid steel is at least comparable to conventional systems with mechanical pumps and certainly less expensive than conventional systems with ejectors.

[00178] The advantages set forth above for the plant 1 according to the invention extend to the degassing method according to the invention.

[00179] The invention thus conceived thereby achieves the intended objectives.

[00180] Obviously, its practical embodiments may assume forms and configurations different from those described while remaining within the sphere of protection of the ^■invention.

[00181] Furthermore, all the details may be replaced by technically equivalent elements and the dimensions, shapes and materials used may be any as needed.

Claims

1. Plant for vacuum degassing liquid steel, comprising: at least one vacuum chamber (2), suitable to temporarily receive liquid steel inside it;

- a vacuum generation system (10), connected to said at least one vacuum chamber (2) via an intake duct (20), characterised in that the vacuum generation system (10) comprises at least two compression stages connected together in series, of which a first compression stage (11) works closer to said at least one vacuum chamber (2) and is formed by one or more screw pumps (110), and a second compression stage (12) works farther with respect to said at least one vacuum chamber (2) to bring the gases at least to atmospheric pressure and is formed by one or more liquid ring pumps (120) and in that said one or more screw pumps (110) are dimensioned to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1:200, if the discharge pressure is comprised between 50 and 120 mbar absolute.

2. Plant according to claim 1, wherein said one or more screw pumps (110) are dimensioned to be able to work with compression ratios comprised between 1:3 to and 1:10, if the discharge pressure is atmospheric, and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios comprised between 1:25 and 1:200, and preferably between 1:70 and 1:90.

3. Plant according to claim 1 or 2, wherein the vacuum generation system (10) is dimensioned to bring the vacuum chamber (2) to a degree of vacuum between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar.

4. Plant according to claim 1, 2 or 3, wherein the vacuum generation system (10) comprises at least One intermediate compression stage, which is positioned between the first stage (11) and the second stage (12) and is connected to them in series, said intermediate compression stage being formed by one or more screw pumps (110) having similar characteristics to those of the first stage ( 11 ) .

5. Plant according to one or more of the preceding claims, wherein one or more of said compression stages are each formed by two or more pumps connected in parallel.

6. Plant according to one or more of the preceding claims, wherein the intake duct (20) comprises a by-pass duct (21) able to exclude from the gas flow the compressor stages formed by screw pumps (110).

7. Plant according to one or more of the preceding claims, wherein each screw pump (110) comprises two screw rotors, kinematically synchronised with each other via electric axis.

8. Plant according to one or more of the preceding claims, comprising at least one filtration device of the water^' used by said one or more liquid ring pumps (120) suitable to remove dust accumulated in the water itself during operation of the pump or a replacement device of the water itself.

9. Plant according to one or more of the preceding claims, wherein, in the section comprised between the vacuum chamber (2) and the vacuum generation system (10) the intake duct (20) comprises a connection branch (28) to the atmosphere equipped with a control valve (23) .

10. Plant according to one or more of the preceding claims, comprising at least one gas filtration device (25) positioned between the vacuum chamber (2) and the vacuum generation system (10).

11. Plant according to claims 9 and 10, comprising at least one shut-off valve (22) that is installed in said intake duct (20) between the vacuum chamber (2) and the filtration device (25), downstream of the branching point of the connection branch (28) to the atmosphere.

12. Method for vacuum degassing liquid steel, comprising the following operating steps:

a) providing at least one vacuum chamber (2) suitable to temporarily receive liquid steel inside it; b) placing liquid steel in said vacuum chamber (2);

c) evacuating the vacuum chamber (2) through a vacuum generation system (10) creating in said chamber a predefined degree of vacuum and maintaining it for a predetermined period of time so as to complete the operation of degassing the liquid steel;

d) bringing again the vacuum chamber (2) to atmospheric pressure and withdrawing the degassed liquid steel;

characterised in that the vacuum evacuation step c) is conducted by means of a vacuum generation system (10) comprising at least two compression stages connected together in series, of which a first compression stage (11) works closer to said at least one vacuum chamber (2) and is formed by one or more screw pumps (110), and a second compression stage (12) works farther with respect to said at least one vacuum chamber (2) to bring the, gases at least to atmospheric pressure and is formed by one or more liquid ring pumps (120) and in that said one or more screw pumps (110) are dimensioned to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1:200, if the discharge pressure is comprised between 50 and 120 mbar absolute.

13. Method according to claim 12, wherein said one or more screw pumps (110) are dimensioned to be able to operate with compression ratios comprised between 1:3 and 1:10, if the discharge pressure is atmospheric, and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios of between 1:25 and 1:200, and preferably between 1:70 and 1:90.

14. Method according to claim 12 or 13, wherein, in evacuation step c) , the vacuum chamber (2) is brought to working at a degree of vacuum between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar.

15. Method according to one or more of claims 12 to 14, wherein said evacuation step c) provides for the direct aspiration of the gases from said vacuum chamber (2) through the said vacuum generation system without a preventive filtration step of the gases, independently of the level of dust concentration in the gases themselves.

16. Method according to one or more of claims 12 to 14, wherein said evacuation step c) provides for the aspiration of the gases from said vacuum chamber (2) through said vacuum generation system with a preventive filtration step of the gases, to reduce the dust concentration in the gases themselves before their passage through the vacuum generation system (10).

17. Method according to one or more of claims 12 to 16, wherein evacuation step c) comprises:

an initial evacuation step cl) wherein the vacuum chamber. (2) is brought from atmospheric pressure up to about 300 mbar using only the liquid ring pumps (120) of the vacuum generation system (10); and

- a final evacuation step c2) wherein the vacuum chamber (2) is brought from the pressure of about 300 mbar to the predefined degree of vacuum also using the screw pumps (110).

18. Method according to one or more of claims 12 to 17, wherein during evacuation step c) , the aspiration capacity of the vacuum generation system (10) is varied to reduce foaming phenomena of the slag in the liquid steel, the aspiration capacity being varied by slowing or excluding one or more of the pumps (110, 120) of the vacuum generation system (10), preferably the liquid ring pumps (120), said variation of aspiration capacity being preferably carried out when the internal pressure of the vacuum chamber (2) is between 300 mbar and 1 mbar.

19. Method according to one or more of claims 12 to 18, comprising a treatment step f) of the water used by said one or more liquid ring pumps (120), said step being preferably carried out during evacuation step c) , said treatment consisting in a filtration of the water from the dust or a replacement of the water itself.

20. Method according to one or more of claims 12 to 19, comprising a mixing step e) of the molten steel at least during the evacuation step