WO2012002897A1

WO2012002897A1 - Process for making a steel melt containing carbide forming elements from iron based raw material and a mineral containing the carbide forming element, an mixture for alloying steel and use of a mineral containing carbide forming elements for alloying a steel melt

Info

Publication number: WO2012002897A1
Application number: PCT/SE2011/050875
Authority: WO
Inventors: Lidong Teng; Mselly Nzotta; Seshadri Seetharaman; Andrei Chychko; Jan-Olov Andersson
Original assignee: Uddeholms AB
Current assignee: Uddeholms AB
Priority date: 2010-06-30
Filing date: 2011-06-29
Publication date: 2012-01-05
Anticipated expiration: 2012-12-30
Also published as: SE1050721A1; WO2012002897A4; TW201217548A

Abstract

It has been proposed to use a carbide forming element in the form of a mineral, e.g. molybdite, MoO₃, to avoid the high cost of ferrocarbide elements as alloy addition agent when melting iron based raw material to steel, but a main drawback has been the high vapor pressure that causes evaporation of the carbide forming element, e.g. MoO₃. These problems are solved by a process for making a steel melt from iron raw material and a carbide forming element, comprising the steps of: a)charging a melting furnace with iron based raw material, b)charging the melting furnace with an amount of an alloy mixture containing a particulate mineral containing a carbide forming element and carbon and, in addition, mill scale and/or CaO or other slag former containing CaO, said alloy mixture preferably being encapsulated in a closed container, said container preferably being placed in the centre of the melting furnace, c)heating the interior of the melting furnace to a temperature of 700 K, d)regulating the energy input to the melting furnace such that the temperature is increased to 900 K during a time period which is adapted to the amount of alloy mixture and the particle size thereof, whereby a ferroalloy containing the carbide forming element will form in the alloy mixture, e)melting the charge to produce a steel containing a high amount of the charged carbide forming element. The process reduces the economic and environmental impacts by minimizing carbide forming element losses to exhaust dust and slag.

Description

PROCESS FOR MAKING A STEEL MELT CONTAINING CARBIDE FORMING ELEMENTS FROM IRON BASED RAW MATERIAL AND A MINERAL CONTAINING THE CARBIDE FORMING ELEMENT, AN MIXTURE FOR ALLOYING STEEL AND USE OF A MINERAL CONTAINING CARBIDE FORMING ELEMENTS FOR ALLOYING A STEEL MELT

DESCRIPTION TECHNICAL FIELD

The present invention relates to a process for making a steel melt containing carbide forming elements from iron based raw material and a mineral containing the carbide forming element, preferably molybdite, a mineral consisting essentially of M0O_3, as the carbide forming element.

It also relates to a mixture comprising a mineral containing a carbide forming element and an oxygen containing material, for example a mineral or mill scale, to be used as a source for alloying elements during steel manufacturing from iron based raw material and the use of molybdite for preparing Fe₂Mo0₄ in a melting furnace during the production of a molybdenum alloyed steel melt.

BACKGROUND ART

The present day steel industry is under extreme strain due to world-wide competition as well as stringent environmental constraints due to global warming. It is realized that drastic changes in steel processes towards environmental improvements cannot be easily accommodated. Modifications of existing processes and design of new process steps can only be enabled using process fundamentals which would lead to a win- win situation due to environmental as well as economic advantages. The demand for raw materials is continually increasing and therefore it is essential to minimize the loss of the valuable metals. Slag, mill scale and dust contain various amounts of iron and alloying elements which provide valuable resources if recycled.

In the following it will be referred to molybdenum, but as is evident for the skilled person the basic principles of the invention, relating to carbide forming elements, may be applied in connection with other metals/elements, wherein similar problems exist. In the case of molybdenum (Mo) in the slag or in dust, the leaching of molybdenum from landfills has turned out to be a challenge, and thus both slag and dust pose an environmental hazard. From economic as well as environmental view points, it is essential that the loss of molybdenum is minimized during an electric arc furnace melting process. In the electric arc furnace practice, alloying with molybdenum to the steel bath is normally carried out by the addition of ferromolybdenum alloy.

Ferromolybdenum alloy is manufactured by refining molybdenum oxide (M0O3) containing mineral. The refining of M0O₃ from mineral form to ferromolybdenum is a bottleneck in today's available assets of ferromolybdenum and a important factor for the cost increase of molybdenum as an alloying element. The mining capacity of M0O₃ in mineral form exceeds the refining capacity. For economic reasons, it has been suggested to substitute molybdenum oxide (M0O3) for the ferromolybdenum alloy. However, the main drawback for the direct M0O₃ addition in electric arc furnace is the evaporation of the M0O3, which has a high vapor pressure at steelmaking temperatures and results in high molybdenum losses to exhaust dust. M. Nzotta, S. Seetharaman and L. Teng reported at the Steel Ecocycle Seminar, May 2006, that the molybdenum loss to the dust could be more than 7 % when M0O3 was used as a raw material in electric arc furnace. Investigations have shown that the molybdenum yield in the steel is approximately 90 % when using M0O₃ as the molybdenum charging source. The evaporation of M0O₃ to the dust is the main cause of loss of molybdenum (about 8.6 % of total molybdenum input). The loss of molybdenum to the slag phase is only about 1.3 % of the total molybdenum input.

The evaporation of molybdenum trioxide has been studied for the past 50 years in connection with the purification of M0O₃ (A.N. Zelikman: Metallurgy of rare metals, Jerusalem, 1966), ferromolybdenum alloy production (L. Norberg and B. Rydstad: Jernkontorets annaler, 160 (1976), No 4, 191), performance of molybdenum containing materials at high temperatures (T. Saburi, H. Murata, T. Suzuki, Y. Fujii and K. Kiuchi: J. of Plasma and Fusion Research, 78 (2002), No 1, 3), and designing of catalysts ( L. Zhang: Applied catalysis A - General, 117 (1994), No 2, 163, and L. Dazhuang, Z. Jianhong, T. Huimin, H. Dongxia: Reaction Kinetics and Catalysis Letters, 62 (1997), No 2, 347). It was found that pure M0O3 sublimes above 973 K, but the evaporation rate becomes very significant beyond the melting point of M0O₃ at 1068 K (T. Saburi et al., supra). Detailed studies of the temperature influence on the vapor pressure of M0O3 were carried out by Blackburn and Johnston (P.E. Blackburn, M.H. Herrick, L.

Johnston: The journal of physical chemistry, 62 (1958), No 7, 769).

Mill scale presents a valuable resource of iron and alloying elements. In some steelworks where forging is a routine forming step, the yield loss from mill scale is quite substantial. The loss of iron through mill scale in the steel production could reach up to 1% wt. From a production economy point of view as well as environmental concern it is desirous to recycle the mill scale. Today, mill scale is used as a source of oxygen and is added to the steel melt for refining of phosphorous. Hot work steels are particularly sensitive to high phosphorous contents. The cost of scrap metal is proportional to the inverse content of phosphorous, which gives a strong economical incentive for an efficient phosphorous refining during the production of steel from scrap metal. In recycling mill scale for phosphorous refinement, about 25% of the mill scale from these steelworks can be recycled. The rest has thus far been deposited.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a process for making a steel melt containing a carbide forming element from iron based raw material and a mineral containing the carbide forming element, while reducing high losses of the carbide forming element primarily to the vapor phase due to the high vapor pressure of the carbide forming element above the melt of raw materials.

In accordance with the present invention, this object is achieved with a process comprising the steps of:

a) charging a melting furnace with iron based raw material,

b) charging the melting furnace with an amount of an alloy mixture containing a

particulate mineral containing a carbide forming element and carbon and, in addition, mill scale and/or CaO or other slag former containing CaO, said alloy mixture preferably being encapsulated in a closed container, said container preferably being placed in the centre of the melting furnace,

c) heating the interior of the melting furnace to a temperature of 600 K,

d) regulating the energy input to the melting furnace such that the temperature is

increased to 900-1000 K during a time period which is adapted to the amount of alloy mixture and the particle size thereof, whereby a ferroalloy containing the carbide forming element will form in the alloy mixture,

e) melting the charge to produce a steel containing a high amount of the charged

carbide forming element.

It has been found that the combination of the above steps in a surprising degree reduces the evaporation of the carbide forming element (X) from the melting charge and thereby the loss of the carbide forming element to the vapor phase, in that through a series of reactions the carbide forming element is directly alloyed into the steel. In a preferred embodiment relating to molybdenum as the carbide forming element, a series of reactions in the mixture of M0O3 and an oxygen containing material, for example CaO or mill scale, will generate Fe₂Mo0₄ or CaMoC^ (ferromolybdenum alloy) in the melting charge and the molybdenum is alloyed into the steel.

The mechanism behind this is the lower Gibbs energies of MXO_n, compounds with the ensuing lowering of vapor pressure such as FeMoO_n or CaMoO_n in case molybdenum is referred to. In the broad concept of the invention, M denotes a metal or alkali metal element such as Fe, Ca, Mg, Ni, and X denotes a carbide forming element such as Mo, Cr, Ti, Nb, Zr, W etc. In addition, a reduction of the losses is positive for the environment, since the amount of carbide forming element leached out from deposited dust and slag will be reduced. Thus, the present invention minimizes the loss of carbide forming element to the dust and slag and is beneficial to the environment and the economy of the process for making alloyed steel melt.

Advantageously, in order to achieve the desired reduction reactions, the molybdite, carbon, mill scale and/or CaO containing slag former are mixed before charging them in the container.

To achieve a desired content of about 1% of molybdenum in the steel melt, the mass of steel scrap usually is on the order of sixty (60) times that of the molybdite.

The melting of the charge is suitably carried out in an electric arc furnace or induction furnace, but the invention shall not be limited by this. It shall be understood that the basic principles of this invention can be applied in connection with any melting furnace using iron based raw material such as steel scrap and iron granules. Relating to melting of steel scrap the invention can be applied in connection with any melting furnace using scrap as a raw material.

The time required for the chemical reactions that takes place during the process is in some ratio in inverse proportion to the particle size of the mixed ingredients, viz. mill scale, carbon and molybdite. Therefore, in order to establish optimal process conditions the particle sizes shall be as small as possible in order to provide good mixing properties and a large reactive surface area whereby the time which is required for the chemical reactions to be completed will be reduced. However, it shall be understood that the retention time has to be adapted to the actual particle sizes so that the retention time at the specified temperature ranges is sufficient in order for the chemical reaction to be able to take place in sufficient degree.

The molybdite suitably has a particle size of essentially less than 1 mm, preferably it is in the form of fine powder grains. The ideal particle size of molybdite should be in the range of 0.1 to 1.0 mm.

Similarly, the mill scale advantageously has a small particle size, preferably of essentially less than 10 cm. Performed tests have shown that mill scale with a particle size of max 3 cm is superior to mill scale with a particle size of 1-7 cm. Mill scale in the form of powder is presumed to provide even better process conditions, but is yet to be evaluated. However, it shall be understood that mill scale may be used in the form and sizes which they fall at the forging mill. Additionally, since mill scale is porous it provides a large reactive area and mix relatively well with carbon and molybdite, particularly if these ingredients are in powder form. Mill scale amount required should be the same as the stoichoimetric amount needed for the formation of Fe2Mo04.

Similarly, the available carbon has a particle size of less than 10 mm. The carbon particle size could be even smaller than 5 mm. During tests, carbon with a particle size of essentially 0.3-1 cm has been used with successful results. Provided that molybdite and mill scale is used with even smaller particle sizes, e.g. in powder form, it is presumed that the particle size of carbon shall be in the same range. Carbon amount required should be in the range of 15-25% of the mass of mixture (mill scale+C+MoOs). This carbon amount should cover both the reduction of Mo03+FeO_x and the expected reoxidation of carbon during scarp melting.

Before CaO acts as a slag former it reacts with M0O3 to form CaMo0₄. During tests, CaO with particle sizes of essentially 1-2 cm have been successfully used. Most preferred, CaO in powder form shall be used.

Another object of the present invention is to provide a new additive for use in the making of a steel melt containing carbide forming element.

That object is achieved in that the additive is a mixture comprising particulate mineral containing a carbide forming element and carbon and additionally either mill scale or at least one CaO containing slag former or both. The mass of the mill scale preferably is about that of the particulate mineral containing the carbide forming element, but the amount has to be determined based on the content of various oxides in the mill scale. Suitably the mineral has a particle size of essentially less than 1 mm and the mill scale a particle size of less than 10 cm, preferably max about 3 cm. Most preferred the particulate mineral and the mill scale is in the form of a fine powder. In case molybdenum is referred to, the particulate mineral is essentially consisting of molybdite, M0O3, and is used for preparing Fe₂Mo0₄ in a melting furnace during the production of a molybdenum alloyed steel melt.

MODE(S) FOR CARRYING OUT THE INVENTION

In the making of steel melts, and especially referring to molybdenum, alloying with molybdenum to the steel bath is normally carried out by the addition of ferromolyb- denum alloy. For economic reasons, it has been suggested to substitute molybdenum oxide (M0O₃) for the ferromolybdenum alloy. However, the main drawback for the direct M0O3 addition in electric arc furnace has been the evaporation of the M0O3, which has a high vapor pressure at steelmaking temperatures and results in high molybdenum losses of up to about 7 % to exhaust dust.

When making a molybdenum alloyed steel melt in accordance with the present invention, the molybdenum source is molybdite, a mineral consisting essentially of following process steps are applied:

a) charging a melting furnace with steel scrap, and

b) charging the melting furnace with an amount of an alloy mixture containing

particulate molybdite and carbon and, in addition, mill scale and/or CaO or other slag former containing CaO, said alloy mixture preferably being encapsulated in a closed container,

c) heating the interior of the melting furnace to a temperature of 600 K,

increased to 900 K during a time period which is adapted to the amount of alloy mixture and the particle size thereof, whereby ferromolybdenum will form in the alloy mixture,

e) melting the charge to produce a molybdenum containing steel melt.

The combination of the above steps reduces the evaporation of M0O3 from the melting charge and thereby the loss of M0O3 to the vapor phase, in that through a series of reactions Fe₂Mo0₄ or CaMo0₄ is formed in the melting charge and the molybdenum is alloyed into the steel. The mechanism behind this is the lower Gibbs energies of FeMoO_x compounds with the ensuing lowering of vapor pressure. In addition, a

reduction of the losses is positive for the environment, since the amount of molybdenum leached out from deposed dust and slag will be reduced. Thus, the present invention minimizes the molybdenum loss to the dust and slag and is beneficial to the

environment and the economy of the process for making a molybdenum alloyed steel melt. It is evident that the same kind of advantages may be gained also in relation to other carbide forming element, such as niob, vanadium, titanium, molybdenum,

chromium and tungsten. Advantageously, in order to achieve the desired reduction reactions, the molybdite and carbon and the possible mill scale and/or CaO or other slag former are mixed before charging them in the steel container. Smaller particle sizes are believed to be

advantageous, whereby the ingredients preferably shall be as small as possible,

preferably in powder form. The primary chemical reactions taking place during the heating are:

Mo0₃₍powder) + C(S ) = Mo0_2(s ) + CO(_gas) (1)

Mo0₂₍s ) + 2C = Mo(p_0wder) + 2CO(_gas) (2)

Other important reactions taking place in the container are:

CaO₍s ) + Mo0₃(s )→CaMo0₄ (3)

CaMo0_{4(s )} + 3C₍s _} = CaO_{(s }} + Mo_{(s }} + 3CO_(g) (4)

Mill scale + C_(s > = FeO_(s > + CO_(g) (5)

Mill scale + C_(s > = Fe_(s > + CO_(g) (6) Mo0_{2(s )} + 2FeO_{(S }} = Fe₂Mo0_{4 (s} > (7)

Fe₂Mo0_{4(s )} + 4C₍s _} = Fe2Mo_{(s }} + 4CO_(g) (8)

As understood by the chemical reaction scheme above, ferromolybdenum are formed in the container by reactions (1), (2), (3) and (4) when molybdite is mixed with carbon and calcium oxide, and by reactions (1), (2), (5), (6), (7) and (8) when molybdite is mixed with carbon and mill scale.

The container may be rigid or flexible, i.e. a steel barrel or a big bag (bulk bag, super sack) of paper or plastics, for example. Preferably the container remains intact during heating at least to a temperature which is high enough for the Fe₂Mo0₄ or CaMo0₄ to have formed, i.e. at around 800-900 K, to prevent or at least minimize the loss of molybdenum to the slag or dust. The melting of the charge is suitably carried out in an electric arc furnace, and as will be described below, the formation of CaMoC^ and Fe₂Mo04 has been observed by high-temperature XRD in the temperature interval 773- 873 K. During subsequent heating to higher temperatures the container shall collapse and release the Fe₂Mo04 or CaMoC^ formed therein into the steel melt.

According to an example, a first amount of steel scrap is charged in the electric arc furnace. The container containing the alloy mixture is charged on top of the first amount of steel scrap, preferably in the center of the melting furnace. A second amount of steel scrap is charged on top of the container and first amount of steel scrap. When electrical power is applied to the furnace, the container will be heated more slowly since it is placed in the centre of the furnace, whereby the retention time in the temperature interval of 600-900 K will become as long as possible, which is advantageous. At normal practice steel melting temperature is reached after about 1 hour of heating and during that time CaMoC^ and Fe₂Mo04 have successfully been formed in the container. Therefore, the retention time in the order of up to about 1 hour is sufficient. If the alloy mixture has small particle sizes a sufficient retention time in the temperature interval of 600-900 K is shorter, preferably in the interval of 1-20 min.

In case an induction furnace is used for the melting process, the sizes of scrap shall be adapted to frequency of the actual melting furnace according to the conventional practice known by skilled persons in the field. According to conventional melting practice in an induction furnace large sized scrap is normally charged in the top of the furnace and starts to melt first, while small sized scrap are charged in the bottom of the induction furnace. The first amount of steel scrap can be a low alloyed small size steel scrap, mostly sheet-steel clippings. The second amount of scrap contains a first portion of small size scrap and a second portion of briquettes of pressed car scrap. Suitably, the mass of the pressed car scrap is about twice that of the small size scrap, and to achieve a desired content of molybdenum in the steel melt, the mass of steel scrap usually is on the order of 50-70 times, preferably sixty (60) times that of the molybdite.

In the example, the molybdite has a particle size of essentially less than 1 mm, while the mill scale to be mixed therewith has a particle size of essentially 1-7 cm and is used in an amount such that the mass of the mill scale is about that of the molybdite. To facilitate a thorough mixing, the mill scale preferably is size separated or reduced to a size, where most of the pieces are smaller than 3 cm. The carbon has a particle size of essentially 0.3-1 cm and present in an amount such that the mass of carbon is between one fourth and half of the mass of the molybdite, and the CaO possibly used for slag forming preferably has a particle size of essentially 1-2 cm.

EXAMPLES

LABORATORY TRIALS

The examples described below illustrate some preferred embodiments of the present invention more in detail. First, the molybdenum yield on reduction of some M0O₃ mixtures in an induction furnace was tested in laboratory experiments and the results are specified in Table I. The chemical composition of the ingredients are specified in table II.

Table I. - Laboratory results on Mo yield in an induction furnace with 16 g and 500 g sample weights

The Ml mixture was aimed at using mill scale and molybdite mineral to provide the transformation of M0O₃ into Fe₂Mo0₄ with further reduction by carbon to form Fe₂Mo. The pure substance of Fe₂Mo0₄ showed the highest molybdenum yield (up to 99 %) during laboratory trials, but the direct usage of initial components (named Fe₂Mo0₄ precursors) is much more attractive in a view of lower material cost.

The M2 mixture is the stoichiometric mixture of carbon and molybdite mineral, which should give pure Mo after reduction. The mixture serves as a reference to compare the Mo yield from different mixtures and to see the influence of other oxides in the mixtures on M0O3 reduction.

The M3 mixture was aimed at using lime and molybdite mineral to provide the transformation of M0O3 into CaMo0₄ above 600 C before the reduction by carbon, which should be more beneficial in the case of fast heating or oxygen presence in a system. The M3 mixture showed lower Mo yield than that of Ml and M2 in the laboratory results. MIXTURE PREPARATION FOR 3 TON TRIALS

The characteristics of the initial components used in the mixture are presented in Table II below. A 10 % excess of carbon was used in the preparation of the mixtures to compensate for possible material loss due to oxidation by air during scrap heating. Table II. - Characteristics of initial components used for alloying mixtures preparation

Indomix, Dolomet, and Alumet are trade names of marketed slag formers having the compositions specified above. All of them contain various amounts of CaO. The weighing of the components of the mixtures was performed on a balance with 1 kg detection limit. The mixing was made in a rotary mixer with a capacity of 75 kg. If it was not possible to put all the initial materials in the mixer for a single batch, the components of a mixture were divided in two portions. No grinding or other size reduction of initial materials was performed during this investigation. However, in the trials specified in Table III, in contrast to the mixtures used in heat numbers Ml -HI and M1-H6, for heat number M1-H2 only small pieces of mill scale (mostly smaller than 3 cm size) were collected to make the mixture.

In total six portions of alloying mixtures were prepared for the experimental trials. The amounts of initial materials, used for the experimental trials are shown in Table III. Table III. - Amounts of initial materials used for experimental trials

CHARGING SEQUENCE

Before starting each experiment, the furnace was preheated due to remaining heat from a previous run. Two types of scrap were used: pressed car scrap and small-sized scrap. Each type of scrap was weighed out within 1 kg accuracy before the experiment. The amount of initial metal scrap for each heat is presented in Table III above.

First, approximately 500 kg of small- sized scrap was charged into the empty hot furnace. Then, the alloying mixture was poured into a steel barrel and covered with a lid. After that, the rest of the small-sized scrap was placed above the barrel, and finally briquettes of pressed car scrap were charged into furnace.

HEAT CONDUCTION AND SAMPLING PROCEDURE

After the furnace was fully charged, the power was applied to a coil up to 2000 kWh. As the melting of scrap started, the power decreased because of scrap melting down. To compensate therefore, new portions of pressed car scrap were added to the furnace. Based on dynamic power observations, it was concluded that the melting of scrap started 30-40 min after charging the small sized scrap. That means a heating rate up to 35-50 K/min, which is 3-5 times higher than that of laboratory trials with 0.5 kg of steel. At the same time, a considerable flame was observed, most probably due to evolution of CO. This was not observed during heats with the same scrap but without addition of alloying mixtures. The sampling of metal was performed using standard lollypop-type sampler. For the first heat, 4 samples were taken, one each 4-6 min. It was found by subsequent chemical analysis that the molybdenum content in steel was stable and did not change with either time or addition of ferrosilicon. After the first heat, only 2 samples of metal were taken for each heat: a first one at 1873 K and a second one - after addition of 10 kg of ferrosilicon - at 1943 K. The temperature of the melt was controlled with a CELOX^® thermocouple sensor. It was noted that most of the samples taken before the silicon addition had poor quality (hollow metal), which resulted in limited possibilities for chemical analysis. Generally, during the investigation the samples were considered after the addition of ferrosilicon.

After the chemical analysis of the samples, the yield of molybdenum was calculated for each heat. As it was not possible to measure the mass of the liquid steel after each trial, the yield of the liquid metal from the scrap was assumed to be equal to be 96 % based on previous observations. During previous heats, a scrap yield range of 95-98 % was observed for the same type of scrap, and because of the similar type of scrap used for each heat, the scrap yield was expected to be approximately the same. The scrap yield value affects the molybdenum yield calculation in the current test.

The results of steel samples analysis and molybdenum yield calculations are presented in Table IV. Table IV. - Results of steel samples analysis and molybdenum yield calculations

The liquid metal mass was estimated based on average metal yield for such types of scrap and furnace. Additionally, the mass of liquid slag was calculated based on known amount of CaO input and CaO concentration in final slag. The slag compositions are presented in Table V. Table V. - Results of slag analysis for the experimental trials

RESULTS AND DISCUSSION

For most of the samples, the molybdenum yield is very close to the laboratory results (Table IV). The carbon content is in the range 0.05-0.08 %, while the charging carbon content was some 0.072 %. That means that a minor decarburization occurred due to oxidation. Also, all the mixtures had 10 % more carbon, which is believed to be consumed by additional oxidation. The analysis of the slag composition showed that a good refining of phosphorous could be obtained in all heats, the content of FeO in the slag being an indicator of this. Heat M1-H6 however deviated from the others in several respects and the results are not considered to be representative for the invention.

In regard to molybdenum, the performed tests have shown that the inventive process is capable of giving a very high yield of molybdenum in the steel. By an alloy mixture containing a stoicio metric mixture of molybdite, carbon and either or both of mill scale and calcium oxide the molybdenum content in the steel can be regulated by adapting the amount of alloy mixture in proportion to the desired content of the alloying element and the yield. By optimization of particle sizes and retention times among other things, a yield in the range of 99% like in the laboratory trials using pure substance of Fe₂Mo0₄ is likely to be obtainable, particularly when alloy ingredients in powder form are used and thoroughly mixed and particularly when mill scale is used for transformation of molybdite in the mineral.

The mixture according to the invention for alloying of steel shall preferably have the following chemical composition:

25 - 65 wt-% of a carbide forming element, preferably molybdite,

10 - 30 wt-% carbon, and either or both of 20-50 wt-% millscale, and

20-40 wt-% CaO and more preferred the components shall be present in the following ranges:

40 - 50 wt-% of a carbide forming element, preferably molybdite,

15 - 25 wt-% carbon, and either or both of

20-40 wt-% millscale, and

20-25 wt-% CaO

Mo0₃ + C mixture

The results show that the molybdenum yield for heat M2-H3 was 92.8 % and for M2- H4 92.6 %, and the results were reproducible. The 93 % molybdenum yield is also in good agreement with that obtained from laboratory test, as shown in Table 1.

Consequently, the molybdenum yield with a mixture of Mo0₃ and carbon can reach 93 %.

Mo0₃ + C + CaO mixture

The molybdenum yield of heat M3-H5 is 92.4 %, and only one heat was tested for this mixture. This yield is close to the laboratory results for 16 g heat size. The molybdenum yield in the laboratory results for 0.5 kg heat size was only 78 %, which must be considered as unreliable. The molybdenum yield with a mixture of Mo0₃, C, and CaO can reach a maximum of 92.4 %. Mo0₃ + C + mill scale mixture

Concerning this Mo0₃ + C + mill scale mixture, three heats were tested with a single charging mixture composition, and the molybdenum yields were quite scattered. The molybdenum yield for heat Ml-was only 89 %, for M1-H2 it was 97 %, and for heat M1-H6 it was 92 %. The possible reasons for the scattered results are summarized below:

1) Heat Ml -HI : Insufficient mixing of Mo0₃ + C + mill scale mixture results in a low yield. The low yield for this heat is probably caused by poor mixing because of mixer overloading and partial loss of materials when they were mixed without lid.

2) Heat M1-H2: Since it was presumed that mill scale of larger size results in poor mixing of mill scale with Mo0₃ and carbon, in heat M1-H2, the size of the mill scale size selected was smaller than that used in heats Ml -HI and M1-H6. For heat M1-H2, mainly small pieces (max 3 cm) were taken during mixture preparation. For the heats Ml -HI and M1-H6 the size distribution of the mill scale was such that some of the pieces were more that 10 cm. Heat M1-H2 shows the best molybdenum yield of 95.9 %. When the content of M0O3 in the slag is considered, it can be seen that the loss of molybdenum to the slag is the least for heat M1-H2, which supports the conclusion that mill scale of smaller sizes provides better process conditions.

The weighing error from the balance has some influence on the calculated molybdenum yield. The detection limit of the balance used was 1 kg. This error will produce ±2 % uncertainty for the molybdenum yield.

Also the scrap yield can affect the molybdenum yield. For example, a variation of 1 % in scrap yield will result in a variation in calculated molybdenum yield of approximately 0.9 %.

The invention also relates to a process for preparing ferroalloy, for example Fe₂Mo0₄, in a furnace comprising the steps of: a) charging the furnace with an amount of an alloy mixture containing a particulate mineral containing a carbide forming element and carbon and, in addition, mill scale and/or CaO or other slag former containing CaO, said alloy mixture preferably being encapsulated in a closed container,

b) heating the furnace to a temperature of 700 K,

c) regulating the energy input to the furnace such that the temperature is increased to 900 K during a time period which is adapted to the amount of alloy mixture and the particle size thereof. By said process, the benefits of the invention can be gained also if a particulate mineral containing a carbide forming element such as molybdite is used in a separate process for production of ferromolybdenum for later use as an alloy ingredient in steel production. INDUSTRIAL APPLICABILITY

The process of the invention makes it possible to avoid the high cost of using ferromolybdenum as alloy addition agent by using the less expensive mineral molybdite in making a molybdenum containing steel melt from steel scrap. The inventive process reduces the economic and environmental impacts by minimizing molybdenum losses to exhaust dust and slag. Moreover, the invention makes it possible to increase the amount of recycled mill scale since mill scale can be used not only for phosphorous refining, but at the same time as an ingredient in an alloy mixture which stabilizes molybdite through a series of reactions during the manufacturing of the steel melt.

As already mentioned above the invention is not limited to the above described examples using molybdenum but may advantageously be used in connection with a number of carbide forming elements. For example a mineral containing vanadium oxide, V₂O₅, or chromium oxide, FeCr₂0₄, Cr₂0₃, may be used.

Furthermore the skilled person realizes that some of the means/steps used in the examples describe above are in no way limiting, but that the skilled person, having knowledge of the basics of the invention, can find a variation of per se known and/or analogues means/steps to obtain the advantages of the invention, without departing from the spirit of the invention.

For example, it shall be understood that the inventive process may be applied to melting processes for production of ferroalloy (e.g. ferromolybdenum) in a separate process which can be added to a steel melt for alloying purposes.

The invention can be applied to oxidizing as well as reducing melting processes, in the reducing process in order to adjust the alloy content of the steel scrap.

Various melting furnaces may be applied where the alloy is processed by a heating from low temperature which provide a temperature increase of the alloy mixture in the container in the interval of 600-900 K during a sufficient retention time. The alloy mixture may be charged in small containers and their form need not be compact, rather a more spread-out form may be advantageous since the alloy mixture will more quickly through heated, providing better conditions for completion of the chemical reactions.

Claims

CLAIMS:

1. A process for making a steel melt containing carbide forming elements from iron based raw material and a mineral containing the carbide forming element, comprising the steps of:

a) charging a melting furnace with iron based raw material,

b) charging the melting furnace with an amount of an alloy mixture containing a

c) heating the interior of the melting furnace to a temperature of 700 K,

increased to 900 K during a time period which is adapted to the amount of alloy mixture and the particle size thereof, whereby a ferroalloy containing the carbide forming element will form in the alloy mixture,

carbide forming element.

A process as claimed in claim 1 , wherein said carbide forming element is molybdenum, charged in the form of molybdite, and said iron based raw material is steel scrap and/or steel granules.

A process as claimed in claim 2, wherein molybdite will form ferro molybdenum through a series of chemical reactions in the alloy mixture.

A process as claimed in claim 2, further comprising mixing the molybdite, carbon, mill scale and/or slag former before charging them in a container.

A process as claimed in any one of claims 1-4, wherein the mass of steel scrap is on the order of 50-70 times, preferably around sixty (60) times, that of the carbide forming element.

A process as claimed in any one of claims 1-5, comprising melting the charge in an electric arc furnace.

7. A process as claimed in any of claims 1-6, wherein the heating of the melting

furnace rate up to 35-50 K/min.

8. A process as claimed in any of claims 1-7, wherein the heating of the melting furnace between 600-900 K is about 1 h.

9. A process as claimed in any one of claims 1-8, wherein the mineral containing the carbide forming element has a particle size of essentially less than 1 mm, preferably the mineral is in powder form.

10. A process as claimed in any one of claims 1-9, wherein the mill scale has a particle size of essentially less than 10 cm, preferably less than about 3 cm, and even more preferred is in powder form.

11. A process as claimed in any one of claim 1-10, wherein the mass of the mill scale is about that of the mineral containing the carbide forming element.

12. A process as claimed in any one of claims 1-10, wherein the carbon has a particle size of essentially less than 10 cm, preferably less than 1 cm and even more preferred is in powder form.

13. A process as claimed in any one of claims 1-11, wherein the mass of carbon is between one fourth and half of the mass of the mineral containing the carbide forming element.

14. A process as claimed in any one of claims 1-13, wherein the CaO has a particle size of essentially less than 10 cm, preferably less than 2 cm, and even more preferred is in powder form.

15. A mixture for alloying of steel comprising a particulate mineral containing a carbide forming element, preferably molybdite, and carbon and additionally either mill scale or at least one slag former containing CaO or both.

16. A mixture for alloying of steel as claimed in claim 15, wherein the mass of the mill scale is about that of the carbide forming element.

17. A mixture for alloying of steel as claimed in claim 16, wherein the mineral

containing the carbide forming element is molybdenum, in the form of molybdite.

18 A mixture for alloying of steel as claimed in claim 17, wherein the molybdite has a particle size of essentially less than 1 mm and the mill scale has a particle size of essentially less than 10 cm, preferably less than 3 cm.

19. A mixture for alloying of steel as claimed in claim 18, wherein the molybdite and mill scale is in powder form.

20. A process for preparing ferroalloy, for example Fe₂Mo0₄, in a furnace using a

particulate mineral containing a carbide forming element as a raw material, comprising the steps of: a) charging the furnace with an amount of an alloy mixture containing a particulate mineral containing a carbide forming element and carbon and, in addition, mill scale and/or CaO or other slag former containing CaO, said alloy mixture preferably being encapsulated in a closed container,

b) heating the furnace to a temperature of 700 K,

c) regulating the energy input to the furnace such that the temperature is increased to 900 K during a time period which is adapted to the amount of alloy mixture and the particle size thereof.