Method of treating steelmaking slag

Abstract

The invention relates to a method of pyrometallurgical smelting of BOF or EAF steelmaking slag according to claim 1. The invention also relates to a granulated slag thus obtained and the use thereof in the production of cement.

Description

[0001] METHOD OF TREATING STEELMAKING SLAG

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method of pyrometallurgical smelting of BOF and / or EAF steelmaking slag. The invention also relates to a granulated slag thus obtained and the use thereof in the production of cement.

[0004] BACKGROUND TO THE INVENTION

[0005] Basic oxygen steelmaking (commonly abbreviated as BOS, BOP, or BOF), also known as Linz-Donawitz steelmaking or the oxygen converter process, is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through a lance over the molten pig iron inside the converter lowers the carbon content of the molten iron and changes it into low-carbon steel. The process is known as “basic” because fluxes of burnt lime or dolomite, which are chemical basis, are added to promote the removal of impurities and protect the lining of the converter.

[0006] A by-product from the basic oxygen steelmaking process is Basic Oxygen Furnace (BOF) steel slag. Around 80-110 kg of BOF steel slag is created per ton of steel produced. A considerable amount of BOF steel slag is currently used in unshaped products. The BOF steel slag or steelmaking slag is typically composed of: about 35-60 wt.% CaO, about 10-15 wt.% SiO2, about 15-35 wt.% total Fe content expressed as FeO (with Fe2O33-10 wt.% and FeO 15-30 wt.%), about 1-5 wt.% AI2O3, about 4-15 wt.% MgO, about 0.3-5 wt.% MnO, and about 0.5-4 wt.% P2O5, and some minor components, especially oxides of Ti, Na, K, and Cr, with < 1 wt.%. The chemical composition of a BOF and EAF steel slag can be determined by XRF as is well known to the skilled person, for example as described in ASTM D5381-93.

[0007] The use of BOF steel slag, however, in cement and concrete industry is limited due to a number of technological barriers, i.e., a low hydraulic activity due to low amorphous content, high amounts of inert phases, and volume instability problems due to the presence of free CaO and MgO.

[0008] In a similar way as with BOF steelmaking slag, the processing and utilisation of steel slag or steelmaking slag from the electric arc furnace (EAF) steelmaking processes is problematic at least in view of the high FeO content which on average might even be higher as compared to BOF steel slag. Commonly the P content in BOF steel slag is higher compared to EAF steel slag.

[0009] An electric arc furnace is a furnace that heats the material by means of an electric arc, which can be supported with the action of the chemical energy provided by the use of oxygen and fuel. The use of the electric arc furnaces allows producing steel starting from a raw material composed of up to 100% metal scraps, which allows to reduce the energy required for the production of steel compared to the primary production of steel from ferrous minerals. The electric arc furnace is an equipment capable of melting different charge mixes, just like the scrap, but also direct reduced iron (DRI) and / or hot briquetted direct reduced iron (Hot Briquetted Iron, HBI). An electric arc furnace operates under oxidizing conditions.

[0010] The EAF steel slag or EAF steelmaking slag is typically composed of, as its main constituents: about 25-35 wt.% CaO, about 12-20 wt.% SiO2, about 30-45 wt.% total Fe content expressed as Fe2O3, about 4-10 wt.% AI2O3, about 4-11 wt.% MgO, about 0.3-5 wt.% MnO, up to about 2.5 wt.% P2O5. The chemical composition of a BOF and EAF steel slag can be determined by XRF as is well known to the skilled person.

[0011] The problem underlying the present invention is to develop a process for treating BOF and / or EAF steel slag which enables the steel residue to be valorised and reused and, at the same time, the risk of leaching of heavy metals to be minimized or eliminated.

[0012] DESCRIPTION OF THE INVENTION

[0013] It is an object of the invention to provide a method of treating steelmaking slag coming from the production of steel by means of BOF and / or EAF to generate a high value slag phase.

[0014] This and other objects and further advantages are met or exceeded by the present invention providing a process according to independent claim 1 and with preferred embodiments in the dependent claims and this description.

[0015] In order to achieve these objects, the present invention proposes, in a first aspect, a method of pyrometallurgical smelting of steel slag, comprising the steps of:

[0016] - feeding basic oxygen furnace (BOF) or electric arc furnace (EAF) steel slag to a smelting unit;

[0017] - smelting the BOF or EAF steel slag under reducing conditions, and preferably by using a reducing agent, and thereby forming a liquid slag phase on top of a liquid ferro alloy phase; reducing conditions is the conversion of the metal oxides (predominantly iron) in the slag to metallic iron via the reduction treatment.

[0018] - adding to the liquid slag phase a predetermined amount of slag formers comprising at least one or more elements from the group of CaO, MgO, AI2O3, and SiO2, and mixed oxides thereof, to produce a slag basicity B3 (based on weight percentage the ratio of (MgO+CaO) / SiO2) in a range of 0.8 to 2.2 and a slag basicity B2 (based on weight percentage the ratio of CaO / SiO2) in a range of 0.7 to 1.40; - measuring the phosphor (P) content in the formed liquid ferro alloy phase and stopping or interrupting the smelting under reducing conditions for tapping of the liquid slag phase when the phosphor content in said liquid ferro alloy phase reaches a level above a predefined threshold set at 0.5 wt.%;

[0019] - and / or measuring the FeO content of the formed liquid slag phase and stopping or interrupting the smelting under reducing conditions for tapping of the liquid slag phase when the FeO content in said liquid slag phase reaches a level below a predefined threshold set at 6 wt.%;

[0020] - separating the liquid slag phase having said slag basicity B3 in the range of 0.8 to 2.2 and a slag basicity B2 in the range of 0.7 to 1.40, and the liquid ferro alloy phase thus obtained by tapping at least a part of said liquid slag phase from the smelting unit. The separating step may take place discontinuously.

[0021] - and wherein the separated and tapped liquid slag phase is granulated, preferably by means of wet granulation.

[0022] The method according to the invention provides for two main output materials each with technical and economic value. Firstly a slag phase is formed that is advantageously directly suitable for use in the production of cement thereby avoiding it to be applied in unshaped products and it enables the replacement of (ground) granulated blast furnace slag (GBFS or GGBFS) and therefore to replace clinker. And secondly a liquid ferro alloy phase is formed having a low phosphor content of less than 0.5 wt.% and usable for subsequent steelmaking. Control of the liquid slag phase composition to desired basicity is by purposely adding slag formers in the appropriate amount and in time to arrive at a slag phase composition having a slag basicity B3 in the range of 0.8 to 2.2 combined with a slag basicity B2 in a range of 0.7 to 1.40, and with preferred ranges herein set forth and claimed.

[0023] The reduction process in the smelting unit leads to a significant reduction of the FeO phases and other metal oxides, e.g., V, Mn, Cr phases, in the processed BOF or EAF steel slag used as feedstock in the smelting unit, and thus also to a required low level of metal oxides in the liquid slag phase tapped from the smelting unit, viz. an amount of FeO of less than 6 wt.%, and preferably of less than 5 wt.%, and more preferably of less than 4 wt.%. Due to the substantial decrease of V, Mn, and Cr in the tapped liquid slag phase which has subsequently been granulated, the potential environmental impact of the granulated slag phase in its application in cement making is mitigated. The method according to this invention doesn’t provide for a maximum recovery of the FeO phases present in the processed BOF or EAF steel slag into a liquid ferro alloy phase, but instead aims at a cautious balance between the two main output materials and providing also a high value slag phase by controlling its composition, and also its morphology following the granulation step, and very importantly having latent hydraulic properties enabling to use it as a raw material for cement making. By carefully controlling both the slag basicity B3 and B2 to the defined ranges when in the smelting unit, the resultant slag phase thus obtained will contain the majority of the phosphorus present in the original BOF or EAF steel slag so that the liquid ferro alloy phase separated from the smelting unit has a phosphor level of less than 0.5 wt.%, and preferably of less than 0.3 wt.% and most preferably of less than 0.15 wt.%, and therewith enabling to use it as input material for steelmaking without the requirement, or at least to a significant lesser extent, of an additional dephosphorisation step commonly practised in steelmaking operations.

[0024] Aiming at maximum FeO recovery from the BOF or EAF steel slag would otherwise lead to a low value slag phase having an unfavourable composition. For example it would lead to a reduction of the phosphorus oxide and adversely increasing the phosphor content in the liquid ferro alloy phase. Said low value slag will have adverse properties such that it requires further and costly processing steps attempting to create a slag phase suitable for use in cement making. More typically such a low value slag phase is mainly handled as waste and sent to landfill. This is seen as a problem in the light of the increasingly stringent regulations on waste management and the increasing costs associated with the disposal thereof, and in the increasingly current perspective of creating a sustainable development model based on reducing the consumption of natural resources and minimizing the production of waste, in which the by-products deriving from industrial processes can be reused rather than disposed of.

[0025] Thus the process according to the invention is very advantageous, as it significantly reduces the annual volumes and costs of disposal of the residues of BOF or EAF steel slag treatment, and it does not create any additional waste streams.

[0026] The liquid slag phase on tapping should have a slag basicity B3 in the range of 0.8 to 2.2 and a slag basicity B2 in the range of 0.7 to 1.40, following which the liquid slag phase is granulated, preferably wet granulated. In an embodiment the slag basicity B3 does not exceed 2.1 , and more preferably does not exceed 2.0. In an embodiment the slag basicity B2 does not exceed 1.2, preferably does not exceed 1.1 , and more preferably does not exceed 1.0. Too high a basicity B3 and B2 results in an increased viscosity, it may result in solidified crystalline phases in the liquid slag phase unable to be converted into an amorphous morphology. Too high a basicity B3 and B2 may also result in free CaO and possibly free MgO in the slag phase, which is undesirable for application of the slag phase in the cement making industry.

[0027] The liquid slag phase and the liquid ferro alloy phase can be tapped off through tapping holes in the smelting unit. In the method, the separated and tapped liquid slag phase is granulated using a fast cooling rate. In an embodiment the separated and tapped liquid slag phase is granulated by means of wet granulation. Quenching using a fast cooling rate makes the slag amorphous and maximizes its reactivity. The aim is to obtain more than 90% glassy solidification, and preferably more than 97%. Granulation eliminates also the nuisance dust associated with conventional cooling of steelmaking slag wherein said slag is tipped in a slag pit and subsequently cooled by pouring water into the slag pit, following which the slag forms large lumps which need to be broken for further processing. The resultant cooled slag has substantially a non-amorphous structure limiting the use thereof in the cement industry.

[0028] The chemical composition of the BOF or EAF steel slag fed or feedstocks into the smelting unit can be measured and is thus known. With that the amount of slag formers to be added can be calculated and added either continuously or continually to the smelting unit to arrive at the defined slag basicity B3 and B2. The slag formers may be pre-heated, e.g. to a temperature above 1000°C, prior to adding these to the smelting unit.

[0029] As the smelting under reducing conditions progresses the FeO content in the slag phase is being reduced, the FeO content in the formed liquid slag phase is being measured and the smelting process may be stopped or interrupted and the liquid slag phase tapped once it is measured and observed that the FeO content is below a predefined threshold set at 6 wt.%. Preferably the predefined threshold is set at 5 wt.%, and more preferably is set at 4 wt.%.

[0030] The measuring of the FeO content in formed liquid slag phase is by taken samples from the slag phase for analysis using measuring techniques or an analysing unit well known to the person skilled in the art and can be done continually, i.e. at certain intervals of for example 10, 15 or 20 minutes, during the smelting process.

[0031] As the smelting under reducing conditions progresses the phosphor content in the molten ferro alloy increases which is considered disadvantageous. Thus also the phosphor content in the formed liquid ferro alloy phase is being measured and when it is measured and observed that the phosphor content is substantially rising and to avoid that it raises to a level above a predefined threshold of 0.5 wt.% phosphor, the smelting process is stopped or interrupted and the liquid slag phase tapped followed by the tapping of the liquid ferro alloy phase. Preferably the predefined threshold for the liquid ferro alloy phase is set at a phosphor content of 0.3 wt.%, and most preferably is set at 0.15 wt.%.

[0032] The measuring of the phosphor content in an liquid ferro alloy is by taking samples from the metal for analysis using measuring techniques or an analysing unit well known to the person skilled in the art and can be done continuously or continually, i.e. at certain intervals of for example 10, 15 or 20 minutes, during the smelting process.

[0033] The method according to the invention can be controlled by either monitoring the variation in the phosphor content or in the FeO content, or both in the phosphor and FeO content.

[0034] By controlling both the formed liquid phase slag basicity B3 and B2 and in the same process measuring and monitoring the development of the FeO content in the slag phase and / or of the phosphor content in the liquid ferro alloy phase and in time stopping or interrupting the reduction process in the smelting unit when a maximum desired FeO content in the formed slag phase is reached and / or a maximum desired phosphor content in the formed liquid ferro alloy phase is reached, the method according to the invention provides for two main output materials each with technical and economic value as set out above.

[0035] In an embodiment of the method it concerns the feeding and smelting of BOF steel slag only in the smelting unit to arrive under reducing conditions at the liquid slag phase having a slag basicity B3 in the range of 0.8 to 2.2 and a slag basicity B2 in the range of 0.7 to 1.40. The initial BOF steel slag fed into the smelting unit has typically a slag basicity B3 of about 3.

[0036] In an embodiment the BOF steel slag processed in according with the invention has a composition of: 35-60 wt.% CaO, 10-15 wt.% SiO2, 15-35 wt.% total Fe content expressed as FeO (with Fe2O33-10 wt.% and FeO 15-30 wt.%), 1-5 wt.% AI2O3, 4-15 wt.% MgO, 0.3-5 wt.% MnO, and 0.5-4 wt.% P2O5, and total of other oxides < 1 wt.% and balance impurities.

[0037] In a preferred embodiment of the method, it concerns the feeding and smelting of EAF steel slag only in the smelting unit to arrive under reducing conditions at the liquid slag phase having a slag basicity B3 in the range of 0.8 to 2.2 and a slag basicity B2 in the range of 0.7 to 1.40. Depending on the operation of the EAF, the initial EAF steel slag fed into the smelting unit has typically a slag basicity B3 in a range of 1.2 to 1.9 and a basicity B2 of more than 1.5, and typically a basicity B2 of more than about 2.

[0038] EAF steel slag originating from an EAF process using as its feedstock only steel scrap material has a low phosphor content as steel, and thus also the steel scrap material feedstock, has a low phosphor content, typically significantly less than about 0.4 wt.% P2O5. Whereas in an ore based EAF process the phosphor content is significantly higher as the iron ore may contain substantial amounts of phosphorous components. The method according to this invention is in particular beneficial for processing ore based EAF steelmaking slag as it enables to process said ore based EAF steel slag into two main output materials each with technical and economic value. To that effect the EAF slag used in the method according to the invention originates from an EAF process using as input material 50 to 90 wt.% of direct reduced iron (DRI) and 10 to 50 wt.% of steel scrap. Typically the DRI feedstock is in the form of pellets or briquettes.

[0039] In an embodiment the EAF steel slag processed in accordance with the invention has a composition of, as its main constituents: 25-35 wt.% CaO, 12-20 wt.% SiO2, 30-45 wt.% total Fe content expressed as Fe2O3, 4-10 wt.% AI2O3, 4-11 wt.% MgO, 0.3-5 wt.% MnO, 0.5-2.5 wt.% P2O5, and total of other oxides < 1 wt.% and balance impurities.

[0040] In an embodiment of the method, a predetermined amount of slag formers is added to the liquid slag phase in the smelting unit and having at least for more than 80 wt.% SiO2, and preferably for more than 90 wt.% SiO2. The addition of SiO2lowers the basicity and also lowers the liquidus temperature of the liquid slag phase facilitating the tapping operation and the sequent granulation.

[0041] Typically BOF steel slag has a MgO content in a range of about 4 to 15 wt.% and EAF steel slag has a MgO content in a range of about 4 to 11 wt.%. Adding too much of MgO as slag former results in an increased MgO content of the liquid slag phase and adversely affecting its subsequent utilisation of the manufacturing of cement.

[0042] Typically BOF steel slag has a CaO content in a range of about 35 to 60 wt.% and EAF steel slag has a CaO content in a range of about 25 to 35 wt.%. Adding more CaO as slag former favours the removal of P from the molten ferro alloy making it suitable for steelmaking. However, CaO in the form of lime stone has a negative CO2footprint. Adding too much of CaO as slag former increases the slag volume, increases the liquidus temperature of the slag and thus requires of higher tapping temperature.

[0043] In an embodiment of the method, the liquid slag phase is tapped at a tapping temperature in a range of about 1300°C to 1600°C to maintain a low slag viscosity. In an embodiment the tapping temperature is at least about 1400°C.

[0044] In an embodiment the liquid slag phase when tapped has a viscosity when measured at 1500°C of less than 1.8 Poise, preferably of less than 1.5 Poise, and more preferably of less than 1.3 Poise.

[0045] In an embodiment the feeding the BOF or EAF steel slag to the melting unit, is onto a residual ferro alloy melt inside said melting unit. The presence of a residual ferro alloy melt inside said melting unit facilitates and accelerates the interaction between the slag phase and the liquid ferro alloy phase and lowers the phosphorus content in the liquid ferro alloy. In an embodiment of the method, the formed and tapped liquid slag phase has a P2O5 content in a range of 0.5 to 6 wt.%, and preferably of 1 to 5 wt.%, and more preferably of 1 to 2.5 wt.%. Phosphor levels in this range do not prevent the utilization of said slag phase once granulated in the making of cement nor in the subsequent concrete products incorporating such a cement as binder. As a resultant of the interaction between the liquid slag phase and the formed liquid ferro alloy phase, the liquid slag phase comprises most of the P2O5present in the initial BOF or EAF steel slag processed in accordance with the invention. In the method according to the invention the smelting under reducing conditions can be stopped and the formed liquid slag phase tapped when it is measured and observed that the phosphor content in the liquid ferro alloy phase is significantly rising.

[0046] In an embodiment of the invention more than 90%, preferably more than 95%, and more preferably more than 98%, of the phosphor present in the initial BOF or EAF steel slag processed in accordance with the invention ends up in the liquid slag phase formed and tapped.

[0047] In an embodiment of the method, the separated and tapping liquid ferro alloy phase has a phosphor content of less than 0.5 wt.%, preferably of less than 0.3 wt.%, and more preferably of less than 0.1 wt.%. It can be subsequently used in a steelmaking process, subjected to secondary metallurgy treatment to adjust its quality and then shaped by casting in a continuous casting plant.

[0048] As a resultant of the smelting process under reducing conditions the amount of Fe in the resultant liquid slag phase after processing the BOF and EAF steel sla is significantly reduced and is not more than 10 wt.% FeO, and preferably not more than 7 wt.%, and most preferably not more than 5 wt.%. The amount of oxides of alloying elements of steel (e.g., MnO, Cr2O3, V2O5, and TiO2) is preferably less than about 5 wt.%, and more preferably less than 3 wt.%. With this the potential environmental impact of the formed slag phase in its application in cement making is mitigated.

[0049] In an embodiment of the method, the smelting under reducing conditions is at a temperature between 1400°C and 1700°C, and preferably between 1500°C and 1700°C.

[0050] In an embodiment of the method, the smelting unit is an electric arc furnace (EAF), a submerged arc furnace (SAF) or an induction furnace (IF).

[0051] Smelting of the BOF or EAF steel slag in a smelting unit under reducing conditions can be done in various ways as known to the person skilled in the art. For example by means of electric heating of the BOF or EAF steel slag and of residual ferro alloy melt present inside the smelting unit as well as of a newly formed ferro alloy melt, and fed by gravity or injecting a carbon- containing reducing agent with gas, preferably with inert gas (e.g. nitrogen), by means of for example a lance into a region close to the boundary surface between the steel slag phase and the ferro alloy melt phase or directly into the ferro alloy melt phase, whereby the carbon is dissolved in the ferro alloy melt and slag are mixed, dissolving the carbon of the reducing agent in the ferro melt and reduction of oxidic iron particles and other metallic oxides of alloying metals of steel (e.g. V, Mn, Cr, and Ti) of the steel slag, with ferro alloy and CO being formed, formation of a liquid slag and wherein in accordance with the invention the composition of the liquid slag is adjusted by the purposive addition of slag formers to produce a slag basicity B3 and B2 within the defined ranges. The addition of carbon, causing an endothermic reaction, serves also to manage the temperature of the system. The residual carbon dissolves in the liquid ferro alloy phase for use as input material in a steelmaking process.

[0052] Heating of the BOF or EAF steel slag in the smelting unit under reducing conditions can take place with resistors, with electric arc, and with a plasma torch.

[0053] In general, carbon-containing materials are used as the reducing agent. As the carbon- containing materials, it may be possible to use, for example, coke, coke ash, graphite, carbon black, peat, coal, biomass or biochar. Coal may be any grade of coal, including lignite, sub- bituminous coal, bituminous coal, steam coal, or anthracite. The carbon-containing material is typically coal powder, and more preferably anthracite powder or coke breeze. The amount of fixed carbon contained in the carbon-containing materials is preferably 50 wt.% or more, more preferably of at least 60 wt.%; the higher the better.

[0054] If carbon is not present in sufficient amount in the melt, the reducing atmosphere may be generated by supplying a reduction gas and / or reduction gas-forming substance.

[0055] In addition to the carbon-containing materials as the reducing agent for the smelting of the BOF or EAF steel slag under reducing conditions, also other reductants can be used to supplement the carbon, in particular aluminium, silicon, and ferrosilicon may be used.

[0056] In an aspect of the invention it relates to an advantageous granulated slag obtained by the method according to the invention, the wet granulated slag comprising, in weight percentage:

[0057] 15 to 45 wt.% SiO2, and preferably 20 to 45 wt.%;

[0058] 25 to 55 wt.% CaO, and preferably 30 to 50 wt.%;

[0059] 4 to 15 wt.% MgO;

[0060] 2 to 12 wt.% AI2O3;

[0061] 0.5 to 6 wt.% P2O5, and preferably 1 to 5 wt.%, more preferably 1 to 2.5 wt.%; up to 6 wt.% FeO, and preferably up to 5 wt.%; and less than 10 wt.%, preferably less than 5 wt.%, of oxides of alloying elements of steel (e.g., MnO, Cr2O3, V2O5, and TiO2), and having a slag basicity B3 in a range of 0.8 to 2.2 and a slag basicity B2 in a range of 0.7 to 1.40, and with preferred B3 and B2 ranges as set out above.

[0062] The wet granulated slag has good hydraulic properties for use in cement making. In an embodiment this is expressed in an F-factor or F-index of >1.4, preferably of F>1 .5, and more preferably F>1 .7. The F-factor is defined as, composition in weight percent:

[0063] F = ( CaO + CaS + 0.5MgO + AI2O3) I (SiO2+ MnO).

[0064] In another aspect of the invention it relates to the use of the separated and tapped slag phase from the smelting unit and subsequently wet granulated, in the production of cement. The cement may be used as a binder in concrete products in particular landscaping products, e.g. pavers and constructions elements.

[0065] EXAMPLE

[0066] The invention is illustrated by the following model calculations, ore-based EAF slag (70% DRI and 30% steel scrap) of average composition (in wt.%): 14.1% SiO2, 29.7% CaO, 38.3% FeO, 9.1% MgO, 4.1% AI2O3, 0.7% TiO2, 2.0% MnO, 0.4% P2O5, balance other metal oxides (e.g. V2O5, Na2O5, K2O, CrOx), and resulting in a B2 basicity of about 2.1 and B3 basicity of about 2.7, is being processed using two different levels of reducing conditions as expressed by the log of the partial pressure of oxygen, also known as the oxygen potential log(pO2), and is a known measure of the chemical potential of oxygen in a system.

[0067] Log(pO2) levels of -14 and -11 have been used representing a reduced degree of reduction. A log(pO2)= -11 (case B) is according to the present invention, whereas case A is an comparative example and representing maximum recovery of the iron from the EAF slag.

[0068] Slag formers (SiO2-source and AI2O3-source) have been added to the EAF slag to arrive at the mentioned slag basicity. Anthracite has been added as reducing agent.

[0069] The results are listed in Table 1. The balance for the “clean” slag composition is made by other metal-oxides, and for the hot metal by other alloying elements. Table 1.

[0070] From the results of Table 1 is can be seen that with a full reduction of the inputted EAF slag (case A using a log(pO2)= -14) into “clean” slag results in a very low FeO content in the slag (viz. about 0.2 wt.%). However, the P-content in the hot metal (HM) or liquid ferro alloy phase is at a very high level (viz. about 0.56 wt.%) such that the HM should be subjected to a subsequent desulphurisation treatment during the steelmaking process. Also to levels of Si and Mn in the HM originating from the reduction of the corresponding metal oxides in the EAF slag are very high. Whereas the EAF slag processed according to the invention (case B) by controlling to B2 and B3 basicity of the steel slag and controlling the smelting under reducing conditions may lead to a hot metal having a favourable low P-content (viz. about 0.04 wt.%) such that there is no need to subject it to a further desulphurisation treatment during the steelmaking process and leading amongst others to considerable cost advantages. Also the levels of Si and Mn in the HM are low. The resultant slag phase (“clean” slag) has a FeO of about 5 wt.% and a P2O5-content of about 0.25 wt.%. These levels of FeO and P2O5in the slag and of the other constituent still render it suitable for use in the production of cement after being granulated using a high cooling rate creating a very high amorphous content following tapping from a smelting unit. Thus two main output materials are obtained each with technical and economic value.

[0071] The amount of “clean” slag and hot metal produced in case A is about 70% slag and 30% hot metal, and for case B about 75% slag and 25% hot metal. Thus case B does not achieve maximum recovery of the iron from the EAF slag, but a favourable balance between the amount of slag and hot metal is achieved whereby two main output materials are obtained each with technical and economic value.

Claims

CLAIMS1 . A method of pyrometallurgical smelting of steel slag, comprising the steps of: feeding basic oxygen furnace (BOF) or electric arc furnace (EAF) steel slag to a smelting unit; smelting the BOF or EAF steel slag under reducing conditions, and preferably by using a reducing agent, and thereby forming a liquid slag phase on top of a liquid ferro alloy phase; adding to the liquid slag phase a predetermined amount of slag formers comprising at least one or more elements from the group of CaO, MgO, AI2O3, and SiO2, and mixed oxides thereof, to produce a slag basicity B3 (based on weight percentage the ratio of (MgO+CaO) / SiO2) in a range of 0.8 to 2.2 and a slag basicity B2 (based on weight percentage the ratio of CaO / SiO2) in a range of 0.7 to 1.40; measuring the phosphor content in the formed liquid ferro alloy phase and stopping or interrupting the smelting under reducing conditions for tapping of the liquid slag phase when the phosphor content in said liquid ferro alloy phase reaches a level above a predefined threshold set at 0.5 wt.%; and / or measuring the FeO content of the formed liquid slag phase and stopping or interrupting the smelting under reducing conditions for tapping of the liquid slag phase when the FeO content in said liquid slag phase reaches a level below a predefined threshold set at 6 wt.%; separating the liquid slag phase having said slag basicity B3 in the range of 0.8 to 2.2 and a slag basicity B2 in the range of 0.7 to 1.40, and the liquid ferro alloy phase thus obtained by tapping at least a part of said liquid slag phase from the smelting unit; and wherein the separated and tapped liquid slag phase is granulated, preferably by means of wet granulation.

2. Method according to claim 1 , wherein the slag formers are added to produce a slag basicity B2 in the range of 0.7 to 1.2, preferably in a range of 0.7 to 1.1 , and more preferably in a range of 0.7 to 1.0.

3. Method according to claim 1 or 2, wherein the liquid slag phase is tapped at a tapping temperature in a range of 1300°C to 1600°C, and preferably in a range of 1400°C to 1600°C.

4. Method according to any one of claims 1 to 3, wherein feeding the BOF or EAF steel slag to the melting unit, is onto a residual ferro alloy melt inside said melting unit.

5. Method according to any one of claims 1 to 4, wherein feeding the smelting unit is with EAF steel slag only.

6. Method according to any one of claims 1 to 5, wherein feeding the smelting unit is with EAF steel slag originating from an ore based EAF process, preferably from an EAF process using as input material 50 to 90 wt.% of direct reduced iron (DRI) and 10 to 50 wt.% of steel scrap.

7. Method according to any one of claims 1 to 6, wherein feeding the smelting unit is with EAF steel slag having a composition of, as its main constituents (in weight percent): 25- 35% CaO, 12-20% SiO2, 30-45% total Fe content expressed as Fe2O3, 4-10% AI2O3, 4-11% MgO, 0.3-5% MnO, 0.5-2.5% P2O5, and balance total of other oxides < 1% and balance impurities.

8. Method according to any one of claims 1 to 7, wherein the FeO content in the formed liquid slag phase is below a predefined threshold set at 5 wt.%, and preferably set at 4 wt.%.

9. Method according to claim 8, wherein said formed liquid slag has further a P2O5content in a range of 0.5 to 6 wt.%, and preferably in a range of 1 to 5 wt.%.

10. Method according to any one of claims 1 to 9, the phosphor content in the liquid ferro alloy phase is above a predefined threshold set at 0.3 wt.%, and preferably set at 0.15 wt.%.

11. Method according to any one of claims 1 to 10, wherein the separated liquid ferro alloy phase has a phosphor content of less than 0.5 wt.%, and preferably less than 0.3 wt.%, and more preferably of less than 0.15 wt.%.

12. Method according to any of claims 1 to 11 , wherein the smelting under reducing conditions is at a temperature between 1400°C and 1700°C.

13. Method according to any one of claims 1 to 12, wherein the smelting under reducing conditions is using a reducing agent, preferably the reducing agent is a carbon-containing material.

14. Method according to any one of claims 1 to 13, wherein the smelting unit is an electric arc furnace (EAF), a submerged arc furnace (SAF), or an induction furnace (IF).

15. A wet granulated slag obtained by the method according to any one of claims 1 to 14, comprising:15 to 45 wt.% SiO2;25 to 55 wt.% CaO;4 to 15 wt.% MgO;2 to 12 wt.% AI2O3;0.5 to 6 wt.% P2O5, preferably 1 to 5 wt.% P2O5; up to 6 wt.% FeO, preferably up to 5 wt.% FeO; less than 10 wt.% of oxides of alloying elements of steel, and having said slag basicity B3 in the range of 0.8 to 2.2 and a slag basicity B2 in the range of 0.7 to 1.40.

16. Use of the wet granulated slag phase according to claim 15 in the production of cement.

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