Agent for desulphurising molten metal

Abstract: The invention relates to a desulphurising agent for molten metal, especially pig iron, which consists of coated particles of magnesium. The object of the invention is to provide a highly effective desulphurising agent that does not suffer from problems of hygroscopicity and abrasion and desegregation of the components during storage and transport. The invention provides a desulphurising agent in the form of coated particles of magnesium having a first coating in contact with the magnesium of silica of finely divided structure in colloidal and/or amorphous and/or crystalline and/or fumed form. This coating acts as lubricating agent and slag liquifying agent for further coatings. The further coatings may be of highly reactive desulphurising agents and further protective coatings and coatings containing fluxes and agents to remove nitrogen and/or oxygen may also be applied.

Description: The present invention relates to an agent for desulphurising molten metals, for example, of liquid pig iron, consisting of granular magnesium particles, each of which comprises a plurality of coatings. Desulphurising agents which consist of a mixture of finely divided magnesium particles and further substances of desulphurising effect, like lime, calcium carbide, limestone and dolomite, are known, e.g. from German Patent No. 3000927 and from U.S. Patent No. 4182626. It is a disadvantage of these desulphurising agents that there occur problems in storage and transport, e.g. by a separation of the agent into its components. There is furthermore the danger that the magnesium of the agent reacts with humidity, resulting in the formation of hydrogen. Problems may also result from the introduction of the desulphurising agent into the liquid molten metal due to the violent reaction of the magnesium with the molten metal. There have been proposed other desulphurising agents containing magnesium, for avoiding one or more of the above described disadvantages. From the European Patent Application No. 0257718 there is known a desulphurising agent consisting of substances like lime, calcium carbide, magnesia, CaAl₂O₃, Al₂O₃ and mixtures of these substances, which are impregnated with magnesium. By this means the danger of separation into the components during transport, storage and handling may be reduced. Furthermore, a more effective desulphurisation, a better concurrence from charge to charge and a reduced metal discharge result. However, it is a disadvantage of this desulphurising agent that the different grains sizes can lead to segregation. A further disadvantage of this desulphurising agent is that the impregnated magnesium is unprotected at the outside, that there is the danger of abrasion so that pure magnesium dust can occur besides granular Mg-mixtures. The magnesium furthermore is located at the outside of the grain so that under the action of humidity the development of hydrogen may occur. In U.S. Patent 4137072 there is described a desulphurising agent which may consist of magnesium and one or a plurality of substances selected from the group of MgO, lime and alumina. By adding binder a moulded product is formed of these components, which may be present in the form of grains, granules, pellets, balls and the like. This known desulphurising agent is said to have a better desulphurising effect over a conventional desulphurising agent of calcium carbide. The indicated components MgO, lime and alumina are to act as diluent for the magnesium in the desulphurising agents, in order to inhibit or minimise the undesired evaporation of the magnesium. They are, furthermore, intended to weaken the violent reaction of the magnesium. The disadvantages of these desulphurising agents consist in that the reduction of the MgO by carbon or any other deoxidant cannot be performed on an industrial scale at RE-temperature (= pig iron temperature), which was already tested with aluminium as the deoxidant. The disadvantages of this desulphuriser further consist in that pure magnesium only exists after the reduction with, e.g. C or Al, and thus can only react thereinafter. Within the short period of injection and rising in the metal bath this desired reaction can only take place insufficiently. There are furthermore known desulphurising agents in which the magnesium particles have coatings of various salt mixtures. In this respect, e.g. British Patent Application No. 2029457, European Patent Application 0058322 and U.S. Patent 4457775 are referred to. The salts mostly are chlorides of a composition which becomes fluid at the same time as magnesium. The application is e.g. performed such that in a salt bath magnesium is molten and stirred and that via a spraying plate the materials are distributed in their liquid state. The results are magnesium balls bearing up to 15% of a salt envelope. This salt envelope is intended to attenuate the material's inclination to explode. These known desulphurising agents have the disadvantage of separating halogen during the reaction. Furthermore, they have variable hygroscopicity, which leads to the reaction of the magnesium with atmospheric moisture which is incorporated into the salt envelope. Storage can furthermore lead to agglomerates which do not allow a purposeful injection with low blowing-in rates. Due to the low melting point of the salt envelope there occur occasional blockages in the injection lances. In European Patent Application No. 0292205 there is disclosed a desulphurising agent in which finely divided magnesium particles exhibit a first coating of a hydrophobic compound, in particular oil, and a second coating of finely divided refractory material. The refractory material of the second coating can consist of one or several substances, selected from the group of alumina, magnesia, silica, titanium oxide, lime, dolomite, calcium carbonate, calcium aluminates, other refractory aluminates, refractory silicates or aluminosilicates. The second coating of refractory material can consist of a first and a second coating itself, the nature of the coatings being identical or different. It is a disadvantage of this desulphurising agent that the coatings substantially are no desuphurising agents, but only ballast substances. In European Patent Application No. 0328270, there is used as hydrophobic compound e.g. epoxide resin or novolak resin for the first coating and soda ash for the second coating. The use of the mentioned organic substance proved to be unfavorable, since during the application a solvent or a curing agent is necessary which can be injurious to the health of the personnel when using the desulphurising agent. Furthermore, the efficiency of the desulphurising agent is not improved either. From German Offenlegungsschrift No. 2618024 there is known a steel treating agent in the form of briquettes which consist of a plurality of layers, wherein the outer layer can comprise deoxidants and the layers underneath can comprise agents for removing nitrogen and sulphur. The disadvantages of this agent are to be seen in that the material cannot be conveyed pneumatically because of the too coarse grain structure and that it must be used in an immersing process which is not suitable for analysis. Finally, it is known from DE-C-3831831 to provide a layer of silica on granules of magnesium and of calcium carbide with an intermediate oil layer between the magnesium or calcium carbide and the silica layer whereby a free-flowing, homogeneous and coated desulphurising agent is obtained. In the steel production of today and the quality requirements connected therewith, it is substantially the accompanying substances nitrogen and sulphur which are unwanted in the steel analysis and they should already have been removed during the pig iron desulphurisation. For economical reasons it is disadvantageous in this process that in the course of the slag removal step iron losses accrue in amounts corresponding to those of the slag. It is a disadvantage of the known desulphurisation process consisting of a mixture of calcium carbide and magnesium that the obtained reaction slag contains high citric acid-soluble portions and thus can only be stored in a special deposit. For improving its desulphurising effect this mixture lacks the conditioning agent, e.g. SiO₂, which, due to its slag-liquifying effect, has a positive influence on the sulphur absorption capacity of the slag. It was the problem of the invention to develop a desulphurising agent of the initially-mentioned kind in a manner that the described disadvantages could be avoided. In particular, it is intended to reduce, with equally good or improved desulphurising effect, the danger of a separation into the components and of the abrasion of the Mg-grain during transport as well as the conveyance problems if there is a component of finely-divided matter and the danger of a metal dust explosion, to avoid the reaction of the magnesium particles with humidity and to achieve a smooth course of the reaction when the desulphurising agent is blown into the molten metal. In addition to the desulphurisation, there are, simultaneously, possible with the novel desulphurising agent, further metallurgical treatments of the molten metals, like removing oxygen and nitrogen and the handling of the desulphurised slag when discharging it from the desulphurisation vessel, in order to minimise the iron losses. Furthermore, the deposit properties are improved and re-use may become possible. Within the scope of the invention these slag losses are reduced by means of slag-conditioning agents, like e.g. by adding fluor spar or SiO₂ -carriers, for liquifying the desulphurised slag. According to the invention an agent for desulphurising molten metal comprises fine granular magnesium particles, each particle having a plurality of coatings, in which the first coating immediately enveloping the magnesium particle is of silicic acid (silica) of very finely-divided structure (and therefore, large specific surface) in colloidal and/or amorphous and/or crystalline and/or fumed form. Because of their finely-divided structure and their large surface, these substances act as lubricating agents and simultaneously as slag liquifying agent for the further coatings. A coating of the indicated substance can, furthermore, be applied on the magnesium particles in a thinner thickness of the layer than the known oil-coating, so that, because of this, the proportion of the further coatings can be increased. The possibility of using highly reactive desulphurising coatings like alkalis and alkaline earths have the advantage that, apart from an improvement of the desulphurising effect, any desired substance combination can be selected and with this an addition of the individual advantages of the respective substances can be achieved. Thus, there exists the possibility, due to the price development of the desulphurising agents on the world market, to combine and use the desulphurising agents which are used as coating/coatings in a manner that there can always be realised that kind of pig iron desulphurisation which is the most economical at that time. The magnesium particles may have further coatings of less reactive desulphurising agents to serve as protective coatings, particularly with regard to the absorbance of humidity and to avoid the formation of hydrogen. Additional further possible coatings made of fluxes, agents for removing oxygen and nitrogen, broaden the field of application of the desulphurising agent according to the invention in an advantageous manner, so that these agents can simultaneously be used for the metallurgical treatment of the molten metal and for conditioning the slag. The highly reactive desulphurising agents are preferably selected from the group of alkaline earths and alkalis, e.g. calcium oxide and sodium carbonate, while the less reactive desulphurising agents are selected from the group of the alkalis and alkaline earths, like e.g. calcium borate, calcium carbonate, Ca(OH)₂, calcium aluminate and sodium borate. If desired, the magnesium particle may have further coatings containing one or more fluxes and/or one or more agents for the removal of nitrogen. The flux is preferably selected from the group of colemanite, fluorspar, cryolite and SiO₂, while the deoxidant is preferably selected from the group of aluminium, calcium-silicon and carbons and magnesium. The agent for removing nitrogen is preferably selected from the group of titanium, aluminium, vanadium, nickel, sodium and magnesium. The size of the granular magnesium is preferably in the range 0.15 to 1.00 mm and the proportion of the magnesium in the coated particle of the desulphurising agent should be from 1% to 90% by weight and is preferably in the range from 15% to 80% by weight. The proportion of the ingredients of the first coating in the coated particle is preferably from 0.5 to 5% by weight. The finely divided silica in the colloidal and/or amorphous and/or crystalline and/or the fumed form preferably has a particle size within the range 0.1 to 5 µm. The desulphurising agent preferably has a grain size of less than 60 µm and is present in the coated particle in an amount from 5 to 25% by weight. However, where a less reactive desulphurising agent is employed the grain size may be less than 90 µm and the agent preferably is present in an amount up to 35% by weight. Where a flux coating is used, the proportion of flux in the coated particle is preferably from 1 to 25% by weight and the grain size of the flux is less than 40 µm. Where a deoxidant coating is used, the proportion of deoxidant in the coated particle is also preferably from 1 to 25% by weight and the grain size of the deoxidant is also less than 40 µm. Where a coating of an agent to remove nitrogen is used, the proportion of that agent in the coated particle is preferably also from 1 to 25% by weight and the grain size of the agent is also less than 40 µm. The preferred desulphurising agent used is a free-flowing soft quicklime. The grain size of this soft quicklime is within the range of < 30 µm, preferably within the range of < 16 µm. It is considered to be an advantage of the invention that the problems occurring with transport, storage and use of magnesium-containing desulphurising agents are solved and that due to the use of pure desulphurising agents as coatings, an improved desulphurising effect can be achieved. Furthermore, there is the possibility to select any combination of substances through which an addition of the individual advantages of the respective substances can be achieved. By using silica of very finely divided structure and large specific surface, of colloidal and/or amorphous and/or crystalline and/or fumed form as first coating layer the proportion of the metallurgically active layers can be increased. The thus possible yield of coatings of fluxes and of agents for removing oxygen and nitrogen broadens the field of application of the desulphurising agent in an advantageous manner. The possibility of different kinds of substance combinations provides the advantage that the slag from the reaction products can be conditioned such that improved deposit properties result. Apart from this the slags conditioned such can be re-used in the process for pig iron production. In the following the invention is explained by way of examples. EXAMPLE 1 A desulphurising agent on the basis of magnesium particles provided with coatings was produced on a plant scale and consisted of the following components in % by weight: The magnesium had an average particle size of 0.5 mm, the colloidal silica had an average particle size of 0.2 µm. The free-flowing soft quicklime and the aluminium each had an average grain size of 90% < 100 µm. The magnesium and the colloidal silica were mixed in a high energy mixer (Eirich mixer) for a period of 5 minutes. Subsequently, there was at first added the free-flowing soft quicklime and the mixing was continued for a period of 5 minutes. Finally, the aluminium was added and stirring was continued for another period of 5 minutes. The degree of protection against the absorption of water, allowed to those magnesium particles having a coating of colloidal silica and the further coatings of free-flowing soft quicklime and aluminium, was estimated by immersing the coated particles into water and by measuring the formation of hydrogen. The formation of hydrogen was found to be = 0.5 l/kg · h. A comparative test showed that magnesium particles without coatings exhibited gas formation of = 1 1/kg · h. The degree of abrasion during the conveyance of the magnesium particles having a coating of colloidal silica and the further coatings of free-flowing soft quicklime and aluminium, in a conveying conduit of 120 m of length was determined to be 14% by weight. A comparable desulphurising agent which did not have a coating of colloidal silica exhibited an abrasion of 23% by weight. With the desulphurising agent according to the invention, pig iron charges were desulphurised which had an initial temperature of 1320°C. The results are shown in Table 1. Explanations 1) ⊘ = average 2) amount of pig iron = amount of the respective pig iron charge in t 3) de-S-amount = amount of desulphurising agent in kg/charge 4) S₀ = initial content of sulphur in the pig iron smelt in % 5) S₁ = final content of sulphur in the pig iron smelt in % after the treatment 6) de-S-amount kg/t RE = specific amount of the desulphurising agent in kg/t of pig iron 7) calculated kg Mg/t RE = calculated amount of Mg/t of pig iron 8) S₀/S₁ = ratio of the initial sulphur content to the final sulphur content 9) 10) 11) K-value = That amount of desulphurising agent which is necessary to achieve desulphurisation 1.0 (100%); also rise in the consumption curve of the desulphurising agent. EXAMPLE 2 A desulphurising agent on the basis of magnesium particles provided with coatings was produced on a plant scale and consisted of the following components in % by weight: The magnesium had an average particle size of 0.5 mm, the colloidal silica had an average particle size of 0.2 µm. The free flowing soft quicklime and the fluor spar each had an average grain size of 90% < 40 µm. The magnesium and the silica were mixed in a high energy mixer (Eirich mixer) for a period of 5 minutes. Subsequently, there was at first added the free-flowing soft quicklime and the mixing was continued for a period of 5 minutes. Finally, the fluor spar was added and stirring was continued for another period of 5 minutes. The coated magnesium particles were separated from the non-adhering particles of colloidal silica, soft quicklime and fluor spar. The proportion of the non-adhering particles was determined to be 11% by weight. A comparable desulphurising agent, which was not provided with a coating of colloidal silica, exhibited 18% by weight of non-adhering particles. The degree of protection against the absorption of water, which is allowed to those magnesium particles having a coating of colloidal silica and the further coatings of free-flowing soft quicklime and fluor spar, are estimated by immersing the coated particles into water and by measuring the formation of hydrogen. The formation of hydrogen was determining to be 0.5 1/kg · h. A comparative test showed that magnesium particles without any coatings exhibited a gas formation of = 1 1/kg · h. The degree of abrasion when conveying magnesium particles with a coating of colloidal silica and with further coatings of free-flowing soft quicklime and fluor spar, in a conveyor conduit of 120 m of length, was found to be 12% by weight. A comparable desulphurising agent which did not have a coating of colloidal silica exhibited an abrasion of 21% by weight. With the desulphurising agent, according to the invention, pig iron charges having an initial temperature of 1320°C were desulphurised. The results are shown in Table 2a. In Table 2b there are furthermore, shown the respective amounts of slag in kg/t RE (pig iron) and the pig iron losses in kg/t RE (pig iron). EXAMPLE 3 A desulphurising agent on the basis of magnesium particles provided with coatings was produced on a plant scale and consisted of the following components in % by weight: The magnesium had an average particle size of 0.5 mm, the colloidal silica had a average particle size of 0.2 µm. The lime and the soda each had an average grain size of 90% < 100 µm. The magnesium and the colloidal silica were mixed in a high energy mixer (Eirich mixer) for a period of 5 minutes. Subsequently, there was at first added the lime and the mixing was continued for a period of 5 minutes. Finally, soda was added and mixing was continued for another period of 5 minutes. The degree of protection against the absorption of water, allowed to those magnesium particles having a coating of colloidal silica and the further coatings of lime and soda, was estimated by immersing the coated particles into water and by measuring the formation of hydrogen. The formation of hydrogen was found to be < 0.5 l/kg · h. A comparative test showed that magnesium particles without coatings exhibited a gas formation of = 2 1/kg · h. The degree of abrasion during the conveyance of the magnesium particles having a coating of colloidal silica and further coatings of lime and soda, in a conveying conduit of 120 m of length, was found to be < 5% by weight. A comparable desulphurising agent which was not provided with a coating of colloidal silica exhibited an abrasion of 15% by weight. With the desulphurising agent according to the invention pig iron charges with an initial temperature of 1320°C were desulphurised. The results are shown in Table 3.

Claims: Agent for desulphurising molten metals, consisting of granular magnesium particles, wherein each of said magnesium particles has a plurality of coatings, characterised in that the first coating immediately enveloping the magnesium particle is of silica of very finely divided structure in colloidal and/or amorphous and/or crystalline and/or fumed form and the structure of the further coatings consists in one or a plurality of highly reactive desulphurising agents. Agent according to Claim 1, characterised in that the magnesium particles additionally comprise one or more coatings of less highly reactive desulphurising agents. Agent according to Claim 1 or 2, characterised in that the magnesium particles additonally comprise further coatings of one or more fluxes and/or one or more deoxidants. Agent according to any one of Claims 1 to 3, characterised in that the magnesium particles additionally comprise further coatings of one or more agents for removing nitrogen. Agent according to any one of Claims 1 to 4, characterised in that the highly reactive desulphurising agents are selected from the group of calcium oxide and sodium carbonate. Agent according to any one of Claims 2 to 5, characterised in that the less reactive desulphurising agents are selected from the group of the alkalis and alkaline earths, like e.g. calcium carbonate, calcium borate, calcium aluminate and sodium borate. Agent according to any one of Claims 3 to 6, characterised in that the flux is selected from the group of colemanite, fluorspar, cryolite and silicic acid. Agent according to any one of Claims 3 to 7, characterised in that the deoxidant is selected from the group of aluminium, calcium-silicon, carbon and magnesium. Agent according to any one of Claims 4 to 8, characterised in that the agent for removing nitrogen is selected from the group of titanium, aluminium, vanadium, nickel, sodium carbonate and magnesium. Agent according to any one of the preceding claims, characterised in that the proportion of the magnesium in the coated particle is from 1 to 90% by weight. Agent according to Claim 10, characterised in that the proportion of magnesium is from 15% to 80% by weight. Agent according to any one of the preceding claims, characterised in that the proportion of the silica of finely divided structure in the coated particle is between 0.5 and 5% by weight. Agent according to any one of the preceding claims, characterised in that the proportion of the desulphurising agent in the coated particle is from 5 to 25% by weight. Agent according to any of the preceding claims, characterised in that the proportion of the less reactive desulphurising agent in the coated particles is up to 35% by weight. Agent according to any one of the preceding claims, characterised in that the porportion of the flux in the coated particle is from 1 to 25% by weight. Agent according to any one of the preceding claims, characterised in that the proportion of the deoxidant in the coated particle is from 1 to 25% by weight Agent according to any one of the preceding claims, characterised in that the proportion of the agent removing nitrogen in the coated particle is from 1 to 25% by weight. Agent according to any one of the preceding claims, characterised in that the grain size of the silica of very finely divided structure is within the range of 0.1 to 5 µm. Agent according to any one of the preceding claims, characterised in that the grain size of the desulphurising agent is within the range of < 60 µm. Agent according to any one of Claims 2 to 19, characterised in that the grain size of the less reactive desulphurising agent is within the range of < 90 µm. Agent according to any one of Claims 3 to 20, characterised in that the grain size of the flux is within the range of < 40 µm. Agent according to any one of Claims 3 to 21, characterised in that the grain size of the deoxidant is within the range of < 40 µm. Agent according to any one of Claims 4 to 22, characterised in that the grain size of the agent for removing nitrogen is within the range of < 40 µm. Agent according to any one of the preceding claims, characterised in that as desulphurising agent a free-flowing soft quicklime is used. Agent according to Claim 24, characterised in that the grain size of the soft quicklime is within the range of < 30 µm. Agent according to Claim 25, characterised in that the grain size of the soft quicklime is within the range of < 16 µm.

Inventors: FLORIN WOLFRAM, FUCHS EDMUND, HOFFKEN ERICH, JAUNICH HELMUTH, FLORIN, WOLFRAM, FUCHS, EDMUND, HOFFKEN, ERICH, JAUNICH, HELMUTH
Applicants: FOSECO INT, FOSECO INTERNATIONAL LIMITED
Application year: 1990
Main classification: C21C1
IPC codes: C21C1, C22B9