WO2005090614A1

WO2005090614A1 - New desulphurating agents for decreasing sulphur content of iron melts to ultra low level

Info

Publication number: WO2005090614A1
Application number: PCT/HU2005/000030
Authority: WO
Inventors: István TAMÁS
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-03-23
Filing date: 2005-03-23
Publication date: 2005-09-29
Anticipated expiration: 2006-09-23
Also published as: HU0400630D0; HUP0400630A2

Abstract

The invention relates to new desulphurating agents for decreasing sulphur content of iron melts to ultra low level (to 0.001% or lower), wherein said agents contain 30-60 w% of non-metallic and 70-40 w% of metallic components, the non-metallic component being tricalciuminate (3CaO.Al2O3) complex oxide, and the metallic component being a Mg or Ca based alloy constituting stable sulphides (Cas or MgS), and wherein the metallic and non-metallic components from molecular pairs with each other. The invention also discloses a method to produce the desulphurating agents according to the invention, comprising the steps of metering adequate quantity of ingoing materials of each desulphurating agents, then individually mixing each reactive components metered in the first step, crushing and homogenizing the mixture by further mixing, briquetting and optionally preheating the powder obtained, vacuum sintering briquettes at a temperature between 850-1100ºC, and a pressure between 10-6 - 10-5bar, and optionally crushing and milling the sintered desulphurating agents before using.

Description

NEW DESULPHURATING AGENTS FOR DECREASING SULPHUR CONTENT OF IRON MELTS TO ULTRA LOW LEVEL

Technical field

The invention relates to new desulphurating agents to decrease sulphur content of iron melts to an ultra low level (to at least 0,001% or lower), and a method to produce desulphurating agents.

Background art

CaSi(Fe) and CaAI(Fe) alloys are currently used to decrease sulphur content of hot steel batches. CaSi(Fe) alloy in the form of powder is burdened to the steel in a ladle by means of argon carrier gas. Such a solution is described in the article "Purity requirements of steel types X65-X80 ERW on producing pipes for long-distance conduits - I.D. Simpson-Z. Tritsiolis-L.G. More (Dunaferr Mϋ- szaki Gazdasagi Kόzlemenyek, 2002/3 pp. 129-137). Admissible upper limit for sulphur content in these types of steel is 0,003%, therefore these steels belong to the group of steels having extremely low sulphur content.

For burdening CaAI(Fe) to the steel batch there is a widespread method using so called powder-core wire. (Metal Producing - 2001/11.: "Using cored technique for ladle metallurgy").

Both burdening techniques above have an essential problem involving the boiling temperature of calcium (1420 °C) being lower than the 1640 °C initial temperature of the steel, thence the calcium being in metallic state in the melt will suddenly pass into vapour-phase having huge volume in both cases, since none of the alloys contain a component being able to damp or control that vaporising process.

Moreover, the rapidly growing metal-vapour bubbles will be combined with each other and they create great bubbles with a diameter of 1-10 cm causing a very intensive bath motion and a spattering of steel, while spouting out of the bath before the calcium would be utilised. Their calcium content combine the oxygen in the air bringing about a high generation of heat, glare light and dense white fume, consequently, the yield of the calcium will be reduced. Thence, using conventional materials, the yield of the calcium is only between 70 and 80 %.

Further disadvantage of using conventional treating materials is the phenomenon of remaining in the melt modified oxysulphide (mainly CaS.CaO.AI₂0₃) inclusions born during desulphurating and deoxidating, and the presence of these inclusions will also be detectable in the rolled steel.

The process using CaSi(Fe) alloys to remove sulphur from the steel melt always pertains increasing silicon content thereof, since burdening calcium of 1 kg into the steel melt increases the silicon content by 3,1 kg (rate of Si/Ca is 2,1). Considering a calcium yield of 70 % in the case of conventional CaSi(Fe) alloy the rate of Si/Ca may increase above 3. The silicon content increased above a predetermined limit may cause an adverse effect in most steels. Therefore, these steels cannot be desulphurated by conventional CaSi(Fe) alloys.

In case of desulphurating steel melt by CaAI(Fe) alloys one must take increase of its Al content into account. This phenomenon may be adverse, because of lowering the temperature of the melt after treatment due to the cooling effect, and therefore the solubility of Ca will also be lowered and in contrast with Al the calcium evaporates from the steel. Hence, after treatment, the remaining Al content will reach a level of 0,015-0,055 w%, and the original Ca content of 40- 90 ppm may be decreased to a value of 4-24 ppm. According to the literature of the art it is very important to maintain a Ca/AI rate in the steel of at least 0,2 or above. The reason is on the one hand the aim of obtaining liquid 3CaOAI₂0₃ inclusions during further deoxidating Al in the cooling steel melt, on the other hand it was detected, that minute slinglike flaw failures still allowed in the pipes produced by welding will be grown to an impermissible level in case of lower Ca/AI rate than 0,2. Nevertheless, the problem does not arise by desulphurating and deoxidating steels alone. For producing steel by converter process the liquid pig-iron bulks up to the most of the batch (up to about 80 %). Therefore, the sulphur content of the liquid iron is a very important factor, since the sulphur content burdened by insert materials might hardly be decreased in the converter. Due to its solid state, it is impossible to beforehand decrease the sulphur content of the scrap steel insert material (about 20 % of the batch). On the contrary, the sulphur content of the liquid pig-iron may be decreased before poured into the converter. This possibility is to be exploited in any case, since decreasing the sulphur content of the steel melt can only be achieved by decreasing firstly the oxygen content of the steel to a level, in which decreasing of the oxygen and sulphur content may simultaneously commence by formation of modified, spherical and solid oxysulphide inclusions. Formation of oxysulphide inclusions then pertains to both alloying the over measure Al into the steel and low rate dissolution of Ca in the steel melt.

If the sulphur content of the liquid pig-iron has been decreased to an ultra low level ( 0,000 <S% < 0,001) before it was poured into the converter, the burdening of sulphur into the steel by pig-iron batch material of 80% will be less than 0,0008%. Therefore, the above condition of 0,000 <S% < 0,001 (which is a requirement for the steel having ultra low sulphur content) can be achieved if the sulphur content burdened by scrap steel of 20% is lower than 0,0002%. In this case the sulphur content of the scrap steel must be lower than 0,01 %. If it is not the case, also the steel melt has to be desulphurated. It is clear, that a part of the sulphur content in the scrap being above 0,01% must be eliminated. This quantity of sulphur can be decreased by assorting the steel scrap, and this is an operation that repays doing.

To decrease the sulphur content of the liquid pig-iron to an ultra low level a Mg containing material is absolutely necessary. Therefore, and also for the reason that the price of this metal shows a decreasing tendency, the Mg containing materials are more and more frequently used in reducing sulphur content of liquid pig-iron. Nowadays, the most frequently used materials contain 85 w% CaC₂ powder with 15 w% Mg powder, or 80 w% CaO powder with 20 w% Mg powder, respectively.

For both materials there is a problem occurring due to the low boiling point of the Mg (1107 °C), since the Mg powder evaporates suddenly at the treatment temperature (1300-1450 °C) of the iron melt. This phenomenon cannot be damped or controlled by the CaC₂ or CaO powder added to the treating material, because the mm sized granules of the powder are discrete granules having no connection therebetween and these granules are only able to reduce the risk of blocking the injection lance and are desulphurating together with the Mg powder up to their ability limit to do it. Then said granules become superfluous with regard to desulphurization. During injection of CO gas the CaC₂ or CaO powder acts as primary desulphurating agent, not disturbed by the Mg powder. Since this action will be followed by a desulphurization process using injected Mg powder only in order to further decrease the S content, some reduction can be obtained in the amount of CaC₂ or CaO powder used.

Due to sudden vaporisation of Mg the rapidly growing Mg-vapour bubbles will be combined with each other causing a very intensive bath motion and a spattering of steel, while spouting out of the bath before the Mg would be utilised, and their Mg content burns out bringing about a high generation of heat, glare light and dense white fume. Consequently, using conventional materials, the utilisation factor of the Mg is only between 50 and 80 % depending on the temperature and initial S content of the iron melt.

In order to diminish above phenomena it was proposed to use MgSiAI alloys instead of using Mg (HU Patent No. 171 849). It is widely known, that desulphurating ability of Mg may be increased for the pig-iron, if desulphurating process is made by an alloy containing Mg₂Si metal compound instead of Mg. (Tamas, Istvan " Analyses for obtaining the most effective reagent in desulphurating pig-iron at low temperature" , 'Banyaszati es Kohaszati Lapok' - periodical publication, vol. 118. , March 1985. Budapest, pages 117-122). However, all these methods had not resolved the problems described above, consequently, there is a need to provide a reliable solution thereof.

Object of the invention

Therefore, first object of this invention is to eliminate any risk of spouting the melt out of the bath and generation of excess heat, glare light, dense white fume and adverse environmental effects accompanying the use of conventional treating alloys.

Further, the second object of the invention is to provide desulphurating agents the use of which is not more expensive than that of the conventional solutions, but suitable for producing iron melts (liquid steel, liquid pig-iron, cast iron melt) having especially good quality and a S content less than 0,001 %.

Further aspect of the invention is to provide a method to produce desulphurating agents according to the invention at an industrial scale.

I have found, that vacuum sintering of mixtures of certain metallic alloys and calcined limestone (CaO) or burnt dolomite (CaO.MgO) issues in new materials being suitable for elimination of above described problems. This invention is based on this apprehension.

Description of the invention

The invention relates to new desulphurating agents for decreasing sulphur content of iron melts to ultra low level (to at least 0,001 % or lower), and said agents contain 30-60 w% of non-metallic and 70-40 w% of metallic components, the non-metallic component being tricalciumaluminate (3CaO.AI₂0₃) complex oxide compound, and the metallic component being a Mg or Ca based alloy constituting stable sulphides (CaS or MgS), and wherein the least metallic and non-metallic components form molecular pairs with each other.

In this description "%" means mass % if it is not otherwise indicated. For desulphuration of steel said metallic component is a Ca based metallic alloy, and for liquid pig-iron or cast iron melt a Mg based metallic alloy is used.

Desulphurating agent for decreasing sulphur content of steel melt to ultra low level contains advantageously a least metallic component belonging to the least non-metallic component (3CaO.AI₂0₃ molecule), which is a Ca based metallic alloy 3[CaAI₂](Fe), and it contains Ca in a range between 30-40 %, preferably 33-37 %, Al between 40-54 %, preferably 46-50 %, and Fe 6-30 %, preferably 13-21 %. This kind of desulphurating agent (desulphurating agent 1) may be produced according to the reaction scheme (1):

6 CaO+8AI(Fe)[a1]→3CaO.AI₂O₃+3[CaAI₂] (Fe) [a2] (1)

This reaction scheme and further reaction schemes are not chemical equations but denotations only to feature chemical processes occurring. In the reaction scheme (1) [a1] denotes an Al-Fe alloy containing 20 % of Fe and 80 % of Al, [a2] denotes a Ca-AI-Fe alloy containing 35 % of Ca and 48% of Al in the form of a compound CaAI₂, as well as 17 % of Fe. The latter, Fe does not participate in the reaction, but its presence is absolutely necessary, namely - according to my experience - the reaction equation

6 CaO + 8AI = 3CaO.AI₂0₃ +3[CaAI₂] cannot be carried out in the practice.

In a further advantageous embodiment of desulphurating agent for decreasing sulphur content of steel melt to ultra low level according to the invention, said metallic component belonging to the non-metallic component (3CaO.AI₂0₃ molecule) is a Ca based metallic alloy 3[CaSi₂](Fe), and containing Ca in a range between 30-38 %, preferably 32-36 %, Si between 42-53 %, preferably 47-51 %, and Fe 9-28 %, preferably 13-21%. This kind of desulphurating agent (desulphurating agent 2) may be produced according to the reaction scheme (2):

6 CaO+2AI(Fe) [a1]+ 6Si(Fe) [a3]→3CaO.AI₂0₃+3[CaSi₂](Fe) [a4] (2) In the reaction scheme (2) [a1] denotes an Al-Fe alloy containing 20 % of Fe and 80 % of Al, and [a3] denotes a Fe-Si alloy containing 25 % of Fe and 75 % of Si. Therefore the reactive alloy contains 23,84 % of Fe, 57,64 % of Si and 18,52 % of Al. The desulphurating agent as a product [a4] contains 17 % of Fe, and (in the form of CaSi₂ metal compound) 49 % of Si and 34 % of Ca.

In a further advantageous embodiment of the desulphurating agent, said metallic component belonging to the non-metallic component (3CaO.AI₂0₃ molecule) is Ca based metallic alloy 5Ca6Si(Fe), and containing Ca in a range between 48-52 %, Si between 40-44 %, and Fe 4-12%. This kind of desulphurating agent (or desulphurating agent 3) may be produced according to the reaction scheme (3):

6CaO+[2AI+2Ca+6Si(Fe)] [a5] — 3CaO.AI₂0₃+5Ca6Si(Fe) [a6] (3)

In the reaction scheme (3) [a5] denotes an alloy containing 10,4 % of Fe, 16 % of Al, 23,7 % of Ca and 49,9 % of Si. The desulphurating agent as a product [a6] contains 8 % of Fe, and (in the form of 5Ca6Si) 42 % of Si and 50 % of Ca.

In an advantageous embodiment of desulphurating agent for decreasing sulphur content of liquid pig-iron or cast iron melt to ultra low level according to the invention, said least metallic component belonging to the least non-metallic component (3CaO.AI₂O₃ molecule) is a Mg based metallic alloy 3(Mg₂Si)(Fe), or 4(Mg₂Si)(Fe), or 5(Mg₂Si)(Fe), each containing Mg in a range between 56- 60 %, Si between 31-35 %, and Fe 5-13%, but the rate of sizes as well as masses of these metallic components is 3 : 4 : 5. These kinds of desulphurating agent (or desulphurating agent 4, 5 or 6) may be produced according to the reaction schemes (4), (5) and (6):

3(CaO.MgO)+[2AI+3Mg+3Si(Fe)][a7]— 3CaO.AI₂0₃+3[Mg₂Si](Fe)[a8] (4)

3(CaO.MgO)+[2AI+5Mg+4Si(Fe)][a9]-→3CaO.AI₂0₃+4[Mg₂Si](Fe)[a8] (5)

3(CaO.MgO)+[2AI+7Mg+5Si(Fe)][a10]-→3CaO.AI₂O₃+5[Mg₂Si](Fe)[a8] (6) In the reaction schemes (4), (5) and (6) [a7] denotes an alloy containing 11 ,71 % of Fe, 22,6 % of Al, 30,54 % of Mg and 35,15 % of Si. [a9] denotes an alloy containing 11 ,48 % of Fe, 16,7 % of Al, 37,3 % of Mg and 34,6 % of Si. [a10] denotes an alloy containing 11 ,34 % of Fe, 13,1 % of Al, 41 ,4 % of Mg and 34,16 % of Si. Each desulphurating agent as a product [a8] includes a metallic component having the same composition and containing 10,86 % of Fe, and (in the form of Mg₂Si) 32,58 % of Si and 56,56 % of Mg, only the ratio of [a8] to the non-metallic component differs in the three reaction schemes, respectively.

Similarly to desulphurating agents for use in liquid pig-iron or cast iron melt, wherein we varied the ratio of metallic component to the non-metallic component, this ratio may be varied in case of desulphurating agents for use in steel melts as well. For example, desulphurating agents containing as metallic component 5[CaAI₂](Fe) or 5[CaSi₂](Fe) besides the non-metallic component 3CaO.AI₂0₃ may be prepared.

For the new desulphurating agents according to the invention said components are evenly distributed in said desulphurating agent, and a metallic component is attached to each non-metallic component forming of a pair therewith (which can be considered as a metallic - non-metallic pair of molecules). It can be assumed, that these pairs of molecules are attached to each other by its opposite sides. Reactions take place in atomic order, therefore, the dimensional order of this components is in the range between 10 - 30 . 10^"10 m.

This invention also discloses a method for preparing new desulphurating agents.

The method includes the following steps: - metering adequate quantity of ingoing materials of each desulphurating agents, namely: - calcined limestone and metallic alloy (a1), in the case of desulphurating agent 1 , - calcined limestone and metallic alloys (a1) and (a3), in the case of desulphurating agent 2, - calcined limestone and metallic alloy (a5), in the case of desulphurating agent 3, - burnt dolomite and metallic alloy (a7), (a9), or (a10), in the case of desulphurating agent 4, 5 or 6, then - individually mixing each reactive components metered in the first step, crushing and homogenizing the mixture by further mixing, - briquetting and optionally preheating the powder obtained, - vacuum sintering briquettes in a temperature between 850 - 1100 °C, and in a pressure between 10^~6 - 10^"5 bar, and - optionally crushing and milling sintered briquettes of desulphurating agents before using. Cost effective calcined limestone (CaO) or burnt dolomite (CaO.MgO) may be used as ingoing material, instead of using most expensive pure CaO or a mixture of pure CaO and MgO. Metallic alloys reacting with said ingoing materials can be purchased commercially or can be prepared by methods well known in the art. Achieving the method according to the invention the adequate quantity of reagents will firstly be metered depending on the composition of the desulphurating agent to be produced in accordance with reaction schemes (1) - (6), namely metering for the preparation of steel desulphurating agents (agents 1 , 2 and 3) calcined limestone and metallic alloy [a1] in the case of desulphurating agent 1 , metallic alloy [a1] and [a3] in the case of desulphurating agent 2, metallic alloy [a5] in the case of desulphurating agent 3; furthermore, metering to produce pig-iron desulphurating agents burnt dolomite containing CaO.MgO and metallic alloy [a7] in the case of desulphurating agent 4, metallic alloy [a9] in the case of desulphurating agent 5, metallic alloy [a10], in the case of desulphurating agent 6, then mixing each reactive component - calcined limestone or burnt dolomite and appropriate metallic component - metered in the first step, and crushing the particles in the mixture into particles of a size of 1 mm or less and homogenizing thereof. Then briquetting the homogenous mixture of powder obtained by a compressing force between 0,5 - 1 ,5 ton/cm2, and preheating briquettes from room temperature to at most 400 °C before vacuum reductive sintering them at a temperature between 850- 1100 °C, and a pressure between 10^"6 - 10^"5 bar or in equivalent conditions. During crushing and briquetting the mixture it is essential to prevent the components of desulphurating agents of different kind from mixing with each other. Consequently, in a preferred embodiment we use separate production lines for producing each desulphurating agent. The obtained agglomerates, i.e. desulphurating agents according to the invention, are cooled, then optionally crushed and milled the briquettes of desulphurating agents into granules having a maximal particle size of 20 mm, and the crushed material is assorted in two size fractions containing particles of at most 2 mm and between 2-20 mm, respectively. The above mentioned chemical processes according to the different reaction schemes take place during the vacuum sintering step of the method according to the invention. The invention involves all desulphurating agents produced by the method described above. The desulphurating agents according to the invention are solid, they may be broken (crushed, milled), their softening temperature is above 850 °C, in the air they do not ignite even at that temperature, their powder is neither inflammable nor explosive, their colour is between dark grey and black and their density is between 1 ,8 and 2,6 kg/cm³. Contrary to conventional desulphurating agents, the desulphurating agents according to the invention may also be applicable without crushing, i.e. in the form of greater pieces of material, since the reaction takes place slowly, and the component -Ca or Mg vapour - necessary to the desulphurization will be evolved discontinuously or in a moderated manner from the desulphurating agent. Before discharge of the melt into a ladle desulphurating agents having greater pieces or particle fractions sized 2-20 mm must be placed onto the bottom thereof. An agent having particle fractions less than 2 mm might be inputted deeply into the melt by e.g. an injection lance engaging almost the bottom of the ladle. When elaborating my invention I assumed the following reaction and action mechanisms: I have found, that desulphurating various kinds of iron melts the biggest problem to be solved was the sudden evaporation of Ca and Mg. This problem might be solved only by moderating and delaying the process of evolution and vaporization of said materials, making the process to be discontinuous, and thus preventing the Ca and Mg bubbles from growing by aggregation. To achieve this aim the evolution of metallic vapour bubbles must be moderated and made it to be discontinuous in order to obtain so small bubbles containing Ca or Mg metallic vapour, which will be utilized and consumed before a new one would be formed. But the utilization is a time consuming process. It is easy to understand, the bigger a bubble the longer the time period necessary to the utilization, since the chemical reaction responsible for the gradual consuming of the bubble might only come off on the surface of the bubble and it takes place gradually as the radius of the bubble is getting smaller. The largest a bubble the higher its speed of uplift. If the uplift time is shorter than the time necessary to complete deperition of the bubble, this bubble breaks out from the bath, therefore it cannot be utilised completely. Consequently, the goal to be achieved is that the uplift time be longer than the time necessary to the complete deperition of the bubble. In the case of a conventional lance immersion depth of 300 cm, a Ca metal vapour bubble having a diameter of 2 cm has an uplift time of 0,0045 s, according to the Stokes formula. If its diameter is 2 . 10^~3 cm, the uplift time is 4500 s. As the dimension of the bubble is getting smaller so the uplift time is getting longer at a quadratic rate. It is essential for this reason, that the bubble be as small as possible: having a diameter of 10^"9 - 10^~8 m, approximately. I have found, that the small CaS.CaO.AI₂0₃ solid inclusions created at the conventional solutions cannot get larger and cannot uplift onto the surface of the steel bath during treatment and remain in the place of their formation. In turn, if the component moderating and damping the evolution of bubbles is chosen such a manner that it is in liquid state, they certainly will join together, the CaS.CaO.AI₂0₃ inclusions being not able to uplift by itself, and so uplift together also decontaminating the steel from the modified spherical oxysulphide inclusions. According to the equilibrium diagram of the CaO.AI₂0₃ system the melting of the 3CaO.AI₂0₃ starts at a temperature of 1535 °C, thence when this one is chosen, it will be transformed to such a melt at a temperature of 1640 °C, which has a melting point decreasing by joining together with the oxysulphide. A liquid oxysulphide slag having a chemical composition of CaS.CaO.AI₂0₃.3CaO.AI₂0₃ obtained this way is able to grow by virtue of further coagulation, thence it uplifts onto the surface of the steel melt at a gradually increasing speed and decontaminating the steel also from the solid oxysulphides. According to my theory, the reaction of the components achieves during vacuum reduction sintering step as follows: Reactive materials compacted as briquettes transform into metallic and non-metallic molecules, according to the reaction schemes (1) - (6). Reaction does not commence all at once in each part of the material, but always on the surface of a briquette, when the temperature and vacuum pressure reach an adequate value. Among reactive materials CaO and CaO.MgO are always in solid state in the temperature of sintering, therefore its position is fixed. Hence the reactive metallic alloys have to have a melting temperature, that makes it possible these alloys to melt in the temperature of sintering, and becoming liquid, to be able to displace. That two Al atoms reacting with 3 CaO or 3MgO in the different cases of calcined limestone or burnt dolomite, respectively, egress always from the metallic reactive material. Those three O atoms will be captured by Al atoms forming Al₂0₃ compound together, and the latter combines further 3CaO at once. This way the non-metallic 3CaO.AI₂0₃ component of the new desulphurating agent according to the invention is formed. At the same time, the 3Ca atoms or 3Mg atoms becoming disengaged evaporate, and the metal vapour bubble alloys into the molten residue of metallic reagent by means of its rapid displacement. The metallic components of different kinds of desulphurating agents according to the invention are formed at the place of this alloying process, such as 3[CaAI₂](Fe), 3[CaSi₂](Fe), 5Ca6Si(Fe), 3[Mg₂Si](Fe), 4[Mg₂Si](Fe), 5[Mg₂Si](Fe). These desulphurating agents can be differentiated from each other by their typical metallic components. The temperature of sintering has to be chosen in such a way, that the melting temperature of each metallic components be higher of that sintering temperature. Thus the metallic component just being formed at the temperature of sintering solidifies by joining to and forming a pair with a non-metallic molecule developed close beside it just a moment earlier. This pair will be isolated from the following metallic component by a newly created non- metallic component, and the chemical reaction continues up to the end of the transformation of all reactive material into metallic and non-metallic components each having a molecular range. Chemical reaction starting on the surface of the briquettes advances in the direction of the core of the briquette in such a way, that an elevated temperature level advances from the outer shell already solidified and having a higher temperature into the core of the briquette. The period of time necessary to the transformation is determined by time passed between the moment of commence of the process in the surface and meeting the initial temperatures starting on the opposite sides of the briquette. Thence the temperature of all briquette will start rising onto the temperature of sintering. Nevertheless, as I mentioned, the sintering temperature must be lower than the melting point of the metallic component created, since in the case the metallic component becomes molten, said pair of molecules formed with the non- metallic component decomposes and the metallic component segregates and joins further metallic components, its dimension gets bigger, therefore the whole bulk of material will be damaged. Consequently, the following conditions must always be complied with: I initial * I mp reactive alloy < ' sintering ' mp metallic component That is, the temperature of sintering has to be higher than the melting point of the metallic reagent and lower than the melting point of the metallic alloy created. The exact sintering temperature to be applied in the method according to the invention has to be determined experimentally, taking into account above conditions. Well known equilibrium diagrams (describing CaO- Al₂0₃, Ca-Si, Al-Ca, Al-Fe and Mg-Si systems) may assist the person skilled in the art in determining this temperature. For the steel treated by conventional materials a part of modified oxysulphide (CaS.CaO.AI₂0₃) inclusions born during desulphurating and deoxydat- ing the bath remain in the melt, and the presence of these inclusions will also be detectable in the rolled steel. Dissimilarly to conventional materials, desulphurating agents according to the invention not only modify oxysulphide inclusions but also clear away said inclusions from the steel. In my theory this is possible by means of the 3CaO.AI₂0₃ molecule being always present in the desulphurating agents according to the invention, which combines the oxysulphide molecule and the 2(2CaOAI₂0₃) product will melt at 1430 °C completely. This material being highly fluid at the temperature of the treatment (1640 °C) and having a molar weight of 428 is able to solve also the solid CaS having a molar weight of 72. The product of this reaction is a fluid oxysulphide slag CaS.2(2CaOAI₂0₃), which can grow by coagulation, therefore it uplifts from the steel bath with a speed rising in accord with the square function of its radius, then dissolves in the slag floating on the surface of the bath, hence enriching CaS, CaO and Al₂0₃ content of the slag. Consequently, the desulphurating agents according to the invention can clear away said modified, solid oxysulphide inclusions, in contrast with conventional materials. Using the desulphurating agents according to the invention instead of conventional materials the steel purified from the oxysulphides will have enhanced ductile properties like elongation, contraction, impact strength and endurance limit. The application of new desulphurating agents according to the invention offers also a solution of easily making up the shortage relating to evaporation of Ca due to cooling during transportation of the ladle into the casting position as well as in the course of casting after desulphuration of the steel melt. This recovery may be owed to the fact, that desulphurating agents according to the invention do not bring about bath motion and it is very easy to solve burdening them into the steel melt during transportation of the ladle into the casting position as well as in the course of casting, too. Reducing sulphur content of steel melt by the desulphurating agents according to the invention has several advantages: desulphuration takes place in few minutes during drawing, no separate time to do it, the yield of the desulphurating Ca and Mg is practically 100 %, it is not dangerous, no pollution and adverse effects to the health. The invention will now be further described by way of examples. The structures of the desulphurating agents produced in the examples was confirmed by X-ray diffraction procedure. Examples Example 1. Producing desulphurating agent (1) containing 3CaO.AI?Q₃ as non- metallic component and 3FCaAI?l (Fe) as metallic component. Homogenous mixture having a particle size of up to 1 ,00 mm was prepared using 336 kg of calcined limestone and 270 kg of reactive alloy (containing 216 kg of Al, and 54 kg of Fe), and briquetting this mixture at a pressure of 1 ,2 t/cm². Briquettes were preheated at 300 °C, then a vacuum reduction sintering process took place at a temperature of 1050 °C and at a pressure of 3.10^"6 bar. The reaction described by the reaction scheme (1) takes place in this process. This way 606 kg of desulphurating agent containing 44,55 % non- metallic component (270 kg) and 55,45 % metallic component (336 kg) was obtained. Comparative Example 1. A comparative test was made using a conventional desulphurating agent and the agent according to the Example 1. Steel containing 0,1 % C, 0,415 % Mn, 0,186 % Si, 0,010 % S, 0,012 % P, 0,009 % Al, and 0,000 % Ca was treated by a conventional alloy CaAI(Fe) by means of which 0,22 kg/ton of Ca was burdened into the batch. Thus the S content of the steel decreased to 0,003 % from 0,01 %, Al content increased to 0,025 % from 0,009 % and Ca content to 0,0062 % from 0,000 %. Yield of Ca was 72,5 %. On the other hand, above steel was treated by desulphurating agent 3CaO.AI₂0₃ + 3[CaAI₂] (Fe) according to the invention and the quantity of Ca burdened into the batch was the same, namely 0,22 kg/ton. This way the S content of the steel decreased to 0,0008 % from 0,01 %, Al content increased to 0,038 % from 0,009 % and Ca content to 0,0103 % from 0,000 %. Yield of Ca was 99,6 %. C, Mn, Si and P content of the steel has not changed. Example 2. Producing desulphurating agent (2) containing 3CaQ.Al2θ₃ as non- metallic component and 3fCaSi?1 (Fe) as metallic component. Homogenous mixture having a particle size of up to 1 ,00 mm was prepared using 336 kg of calcined limestone and 291 ,5 kg of reductive alloy (containing 54 kg of Al, and 69,5 kg of Fe, and 168 kg of Si) forming metallic component, and briquetting this mixture at a pressure of 1 ,0 t/cm². Briquettes were preheated at 100 °C, then a vacuum reduction sintering process took place at a temperature of 1000 °C and at a pressure of 1 .10^"6 bar. The reaction described by the reaction scheme (2) takes place in this process. This way 627,5 kg of desulphurating agent containing 43 % non-metallic component (270 kg) and 57 % metallic component (357,5 kg) was obtained. Comparative Example 2 A comparative test was made using a conventional desulphurating agent and the agent according to the Example 2. A steel having a composition as in Example 1 was treated by a desulphurating agent according to the Example 2 by means of which 0,22 kg/ton of Ca was burdened into the batch. Thus the S content of the steel decreased to 0,001 % from 0,01 %, Ca content increased to 0,0115 % from 0,000 % and Si content to 0,216 % from 0,186 % . Yield of Ca was 99,55 %. C, Mn, Al and P content of the steel has not changed. Above steel was treated by conventional desulphurating agent CaSi (Fe) and the quantity of Ca burdened into the batch was the same, namely 0,22 kg/ton. This way the S content of the steel decreased to 0,0032% from 0,01 %, Si content increased to 0,232 % from 0,186 %, and Ca content to.0,0058% from 0,000 %. Yield of Ca was 65 %. Example 3. Producing desulphurating agent (3) containing 3CaO.AI?Q^ as non- metallic component and 5Ca6Si(Fe) as metallic component. Homogenous mixture (670 kg) having a particle size of up to 1 ,00 mm was prepared from 336 kg of calcined limestone and 334 kg of reactive alloy (containing 54 kg of Al, 80 kg of Ca, 168 kg of Si and 32 kg of Fe), and briquetting this mixture at a pressure of 1 ,0 t/cm². Briquettes were preheated at 100 °C, then a vacuum reduction sintering process took place at a temperature of 1100 °C and at a pressure of 10^~5 bar. The reaction described by the reaction scheme (3) takes place in this process. This way a desulphurating agent containing 40,3 % non-metallic component 3CaO.AI₂0₃ (270 kg) and 59,6 % metallic component 5Ca6Si(Fe) (400 kg) was obtained. Comparative Example 3. A comparative test was made using a conventional desulphurating agent and the agent according to the Example 3. A steel having a composition as in Example 1 was treated by a desulphurating agent according to the Example 3 by means of which 0,22 kg/ton of Ca was burdened into the batch. Thus the S content of the steel decreased to 0,001 % from 0,01 %, Ca content increased to 0,0111 % from 0,000 % and Si content to 0,205 % from 0,186 % . Yield of Ca was 99,57 %. C, Mn, Al and P content of the steel has not changed. Above steel was treated by conventional desulphurating agent CaSi (Fe) and the quantity of Ca burdened into the batch was the same, namely 0,22 kg/ton. This way the S content of the steel decreased to 0,0032% from 0,01 %, Si content increased to 0,232 % from 0,186 %, and Ca content to 0,0058% from 0,000 %. Yield of Ca was 65 %. In my theory, each desulphurating agent to desulphurate liquid steel described in above Examples has a feature of comprising metallic and non- metallic components being in such an arrangement, that non-metallic molecules isolate metallic molecules being close beside and attached thereto, from which the metal vapour bubble evolutes. Therefore the evolution of Ca metallic vapour bubbles is moderated and discontinuous to obtain so small bubbles, which may be utilized and consumed before a new one would be formed. Consequently, the evolution of a large Ca vapour bubble created by using conventional materials will completely be impossible. A hard evidence of this theory is that in case of using new desulphurating agents the forceful bath motion, said spouting out the bath and a high generation of heat, glare light and dense white fume will fail. Since the Ca metallic vapour does not come in the air, the yield of the calcium will be enhanced, namely the Ca yield is almost 100%. Furthermore, using desulphurating agents according to the invention, the Si/Ca = 3 ratio featuring conventional alloys CaSi(Fe) - taking into account the 70 % yield of the Ca, too - might be reduced to a value of 1 ,4 or 0,84, and therefore the treatment of steel melt by desulphurating agents becomes possible in a wider range due to their lower Si content in comparison to conventional CaSi(Fe) alloys. Above theory and observations relating to the evolution of Ca metallic vapour during desulphuration of steel melt also apply to the evolution of metallic vapour of Mg in the case of liquid pig-iron. The problem concerning sudden evaporation of Mg and its consequences may be solved by the same way. Analogously to the reduction process achieved by Al the non-metallic component here again is the 3CaO.AI₂0₃. CaO and MgO both necessary to this procedure can advantageously be found in the burnt dolomite in a molar ratio of 1 : 1. Si seems to be the most adequate material to bind the Mg reduced by Al because of the above described features relating to the Mg₂Si, and a further reason of having a high melting point (1100 °C) making it able to rise the melting point of the emerging metallic component above the temperature of sintering. Following examples show production of desulphurating agents adapted to desulphurate pig-iron and cast iron melts. Example 4. Producing desulphurating agent (4) containing 3CaO.AI?Q₃ as non- metallic component and 3fMg₂Sil (Fe) as metallic component. A powder mixture (528 kg) having a particle size of up to 1 ,00 mm was prepared by using 289 kg of burnt dolomite and 239 kg of reactive alloy (containing 54 kg of Al, 73 kg of Mg, 84 kg of Si and 28 kg of Fe), briquetting this mixture, then a sintering process took place at a temperature of 850 °C and at a pressure of 1.10^"6 bar. The reaction described by the reaction scheme (4) takes place in this process. This way a desulphurating agent containing 51 % 3CaO.AI₂O₃ non- metallic component (270 kg) and 49 % 3[Mg₂Si] (Fe) metallic component (258 kg) was obtained. (From these metallic components will evolute the metallic vapour bubbles) Comparative Example 4. A comparative test was made using a conventional desulphurating agent and the agent according to the Example 4. Pig-iron melt containing 4,46 % C, 0,68% Mn, 0,73 % Si, 0,07 % P was treated by a conventional desulphurating mixture CaO : Mg (80:20). By this method 0,63 kg/ton of Mg was necessary to burden into the batch in order to reduce the S content to 0, 001 % from 0,057 %. When using desulphurating agent according to the Example 4. only 0,42 kg/ton were needed. Yield of Mg was 99,5 % in the latter case in comparison to conventional solution where it is only 66,3 %. If 0,63 kg/t of Mg is burden to the batch, S content decreases to 0,001 % from 0,057 %. Consequently, the new desulphurating agent is completely suitable for substitute conventional materials, or is even better not only to decrease S content of pig-iron melt, but to produce nodular cast iron, too. Example 5. Producing desulphurating agent (5) containing 3CaO.Al?Q as non- metallic component and 4[Mg₂Sil (Fe) as metallic component. Homogenous mixture (613,86 kg) having a particle size of up to 1 ,00 mm was prepared by using 289 kg of burnt dolomite and 324,86 kg of reactive alloy (containing 54 kg of Al, 121 ,56 of Mg, 112 kg of Si and 37,3 kg of Fe), and briquetting and vacuum reduction sintering this mixture at a temperature of 870 °C and at a pressure of 2 .10^"6 bar. The reaction takes place according to the reaction scheme (5). This way 613,86 kg of desulphurating agent containing 44 % non- metallic component 3CaO.AI₂O₃ (270 kg) and 56 % metallic component 4[Mg₂Si] (Fe) (343,86 kg) was obtained. This material is completely suitable for substitute conventional materials. Comparative Example 5. A comparative test was made using a conventional desulphurating agent and the agent according to the Example 5. Pig-iron melt having a composition as in Example 4. was treated by a desulphurating agent consisting a mixture of CaO : Mg = 80 : 20, by means of which 0,67 kg/ton of Mg was to be burdened into the batch in order to decrease the S content of the melt to 0,001 % from 0,050 %. Yield of Mg was 55,6 %. On the other hand, above melt was treated by desulphurating agent 3CaO.AI₂0₃ + 4[Mg₂Si] (Fe) according to the Example 5., and the quantity of Mg burdened into the batch was only 0,375 kg/ton to achieve the same result of desulphuration. The yield of Mg was 99,3 %. In the case of burdening 0,67 kg/ton of Mg into the batch, the S content of the melt will obviously decrease below 0,001 %. Example 6. Producing desulphurating agent (6) containing 3CaO.AI?O₃ as non- metallic component and δfMggSfl (Fe) as metallic component. A powder mixture (699,784 kg) having a particle size of up to 1 ,00 mm was prepared by using 289 kg of burnt dolomite and 410,784 kg of reactive alloy (containing 54 kg of Al, 170,184 kg of Mg, 140 kg of Si and 46,6 kg of Fe), briquetting this mixture, then a sintering process took place at a temperature of 905 °C and at a pressure of 5.10^"6 bar. The reaction takes place according to the reaction scheme (6). This way a desulphurating agent (699,784 kg) containing 38,6 % 3CaO.AI₂0₃ non-metallic component (270 kg) and 61 ,4 % 5[Mg₂Si] (Fe) metallic component (429,784 kg) was obtained. This material is completely suitable for substitute conventional materials, as it will be confirmed by the following comparative example. Comparative Example 6. A comparative test was made using a conventional desulphurating agent and the agent according to the Example 6. Pig-iron melt having a composition as in Example 4 was treated by a desulphurating agent consisting of a mixture of CaO : Mg = 80 : 20, by means of which 0,56 kg/ton of Mg is to be burdened into the batch for the purpose of decreasing the S content of the melt to 0,001 % from 0,040 %. Yield of Mg was 52,9 %. On the other hand, above melt was treated by desulphurating agent according to the Example 6, and the quantity of Mg burdened into the batch was only 0,299 kg/ton to achieve the same result of desulphuration. The yield of Mg was 99,1 %. Application of new desulphurating agents according to the invention provides several advantages: eliminating any risk of spouting the melt out the bath and generation of excess heat, glare light, dense white fume and adverse environmental effects pertaining conventional treating alloys. Since there is no evaporation of metal vapour into the air, the yield of the desulphurating Ca and Mg is practically 100 %, it is not dangerous, no pollution and environmental drawbacks and adverse effects to the health.

Claims

1. A desulphurating agent for decreasing sulphur content of iron melts to an ultra low level (to 0,001 % or lower), characterised in that said agent contains 30-60 w% of non-metallic and 70-40 w% of metallic components, the non- metallic component being tricalciumaluminate (3CaO.AI₂0₃ ) complex compound, and the metallic component being a Mg or Ca based alloy constituting stable sulphides (CaS or MgS), wherein the least metallic and non-metallic components form pairs with each other.

2. The desulphurating agent for decreasing sulphur content of steel melt to an ultra low level according to claim 1 , characterised in that said metallic component belonging to the non-metallic component (3CaO.AI₂0₃ molecule) is 3[CaAI₂](Fe), a Ca based metallic alloy, containing Ca in a range between 30- 40 %, Al between 40-54 %, and Fe 6-30%.

3. The desulphurating agent according to claim 1 for decreasing sulphur content of steel melt to an ultra low level, characterised in that said metallic component belonging to the non-metallic component (3CaO.AI₂0₃ molecule) is 3[CaSi₂](Fe), a Ca based metallic alloy, containing Ca in a range between 30- 38 %, Si between 42-53 %, and Fe 6-30%.

4. The desulphurating agent according to claim 1 for decreasing sulphur content of steel melt to an ultra low level., characterised in that said metallic component belonging to the non-metallic component (3CaO.AI₂0₃ molecule) is 5Ca6Si(Fe), a Ca based metallic alloy, containing Ca in a range between 48- 52 %, Si between 40-44 %, and Fe 4-12%.

5. The desulphurating agent according to claim 1 for decreasing sulphur content of liquid pig-iron or cast iron melt to an ultra low level, characterised in that said metallic component belonging to the non-metallic component (3CaO.AI₂O₃ molecule) is 3(Mg₂Si)(Fe), or 4(Mg₂Si)(Fe), or 5(Mg₂Si)(Fe), Mg based metallic alloys, each containing Mg in a range between 56-60 %, Si between 31-35 %, and Fe 5-13%.

6. The desulphurating agent according to any preceding claim, characterised in that said components are evenly distributed in said desulphurating agent, and a metallic component is attached to each non-metallic component forming of a pair therewith.

7. The desulphurating agent according to any preceding claim, characterised in that the grain-size of components in said desulphurating agent is in a range between 10-30 . 10^"10.

8. A method to produce desulphurating agent according to any of claims 2 to 5, characterised in that metering adequate quantity of ingoing materials of each desulphurating agents, namely: - for the preparation of the desulphurating agent according to claim 2, calcined limestone and metallic alloy (a1), - for the preparation of the desulphurating agent according to claim 3 calcined limestone and metallic alloys (a1) and (a3), - for the preparation of the desulphurating agent according to claim 4, calcined limestone and metallic alloy (a5), - for the preparation of the desulphurating agents according to claim 5, burnt dolomite and metallic alloy (a7), (a9), or (a10), then - individually mixing each reactive components metered in the first step, crushing or milling the mixture to a size of 1 mm or less, and homogenizing it by further mixing, - briquetting and optionally preheating the homogenous powder obtained, - vacuum sintering briquettes at a temperature between 850 and 1100 °C, and in a pressure between 10^"6 - 10^"5 bar, and, after cooling, - optionally crushing and milling the obtained desulphurating agents before using.

9. The method according to claim 8, characterised in that briquetting said powdered mixture of reactive components by a compressing force between 0,5 and 1 ,5 ton/cm².

10. The method according to claim 8, characterised in that preheating said briquettes to at most 400 °C before sintering.

11. The method according to claim 8, characterised in that crushing and milling said sintered desulphurating agents into granules having a maximal particle size of 20 mm.

12. The method according to claim 11., characterised in that assorting said crushed material into two fractions containing particles of less than 2 mm and between 2 and 20 mm, respectively.

13. The desulphurating agent produced by a method according to any of claims 8 to 12.