WO2001088206A1

WO2001088206A1 - Method for recovering useful metal from slag generated in steel-making process while treating the slag

Info

Publication number: WO2001088206A1
Application number: PCT/KR2000/000496
Authority: WO
Inventors: Oksoo Oh; Sangyoon Oh
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-05-19
Filing date: 2000-05-19
Publication date: 2001-11-22
Anticipated expiration: 2002-11-19
Also published as: KR20020033008A; AU2000247847A1

Abstract

Disclosed herein is a method for recovering useful metals from a slag generated in a steel-making process while treating the slag. The method comprises dropping a molten slag to a horizontally blowing air stream while spraying water to the slag. The slag treated according to the present invention is a chemically stable spheroid which can be reclaimed by being sorted depending on the size.

Description

METHOD FOR RECOVERING USEFUL METAL FROM SLAG GENERATED IN STEEL-MAKING PROCESS WHILE TREATING

THE SLAG

Technical Field

The present invention relates to a method for recovering useful metal from slag. More particularly, the present invention relates to a method for recovering useful metal from slag generated in a steel-making process conducted in a converter while treating the slag. In this method, the useful metal is recovered by passing the slag through a zone to which a blown air stream and water are sprayed, and the same time the remaining slag component, from which the useful metal was recovered, is converted into a stable compound having weathering resistance such that it can be used as an aggregate for concrete and the like.

Background Art

The present invention is related to Korean Patent No. 10-98062, US Patent No. 5,417,738, and Japanese Patent No. 2,521,030, the disclosure of which is incorporated herein by reference.

In a steel-making process, a slag is necessarily generated as a by-product. A blast furnace slag is generated from a process during which pig iron is obtained from iron ore, a raw material. The blast furnace slag is a residue remaining after the pig iron is obtained by introducing iron ore, coke, limestone and the like into a blast furnace, and then combusting the coke to heat and reduce the iron ore. This blast slag is mostly reclaimed in an application, such as a barasu for civil-engineering works, or a calcium silicate fertilizer, etc. The slag is produced from the blast furnace in the amount of about 500 to 1100 kg/ton.

A converter slag is a residue remaining after the pig iron is oxidized in a converter under an oxygen gas atmosphere such that carbon, silicon, and phosphorus, etc. contained in the pig iron are removed to obtain a molten steel. In Korea, the slag generated from the steel-making operation amounts to 4,000,000 tons/year or more. The blast furnace slag is mostly reclaimed, whereas the converter slag containing a large amount of specific components, such as heavy metal components, is disposed of by being buried in the ground without the reclamation. Also, the converter slag is classified as a pollutant harmful to environment, and a method for treating the waste slag in a safe manner is thus required. Meanwhile, the converter slag contains useful metal in a large amount, generally 15 to 25% relative to the amount of the slag, and an operation for the recovery of the useful metal thus needs to be carried out .

In the conventional method for _. recovering and treating useful metal from the converter slag, a slag of about 1600 °C is cooled down in the atmosphere and/or water to be solidified. The solidified slag mass is pulverized in large-sized pulverizers, after which the resulting slag powder is transported to a conveyer which is equipped with a magnet, and where the useful metal is recovered. However, this useful metal-recovering method necessarily requires the installation of a plurality of large-sized pulverizers for pulverizing the slag and magnetic conveyers, and thus is expensive in installation cost and subsequent useful metal-recovering costs . Especially, polluting factors such as dust and noise also occur during the pulverization of the slag, so that the working conditions for workers become deteriorated. In addition, as the powdery slag remaining after the useful metal is recovered therefrom is comprised of CaO as its main component, the treatment of the powdery slag by landfill, etc. will result in problems such as ground sinking, etc.

In an approach to resolve the above problems, we have proposed a method for recovering useful metal contained in a slag as disclosed in Korean Patent No. 10-98062 to which US Patent No. 5,417,738, and Japanese Patent No. 2,521,030 correspond. In the disclosed method, a slag generated from the steel-making process is atomized in water, classified depending on its size, and then sorted into a first group having a higher specific gravity and a second group having a lower specific gravity by means of a blown air stream. Then, the first group of a higher specific gravity is transferred to a f rnace. The method disclosed in these patents comprises atomizing the slag in water, sorting the atomized slag depending on the size and specific gravity into a high-weight slag and a low-weight slag, and sending the group of high-weight slag to a furnace where the useful metal is recovered from the slag. As used in these patents, the term "atomizing in water" is understood to be a process in which the slag is cooled in water and formed into a particulate. Disclosure of the Invention

We have carried out a continued study to solve the problems in the conventional slag treatment and to improve the invention disclosed in the above patents. As a result of that, we have found that, where a converter slag which is in a molten state is passed through a spray zone formed by blowing air containing small water drops at a constant blowing pressure, it is possible to more effectively recover useful metal contained in the slag and also to convert the slag remaining after the recovery of useful metals into a more stable material. Based on this discovery, the present invention has been perfected. That is to say, the present invention includes cooling the molten slag with the sprayed water while bringing the molten slag into a contact with the blown air stream to facilitate the granulation of the molten slag.

It is therefore an object of the present invention to provide a method for recovering useful metal from slag while treating the slag, the method comprising spraying air containing small water drops at a constant blowing pressure to a spray zone; and allowing a converter slag in a molten state to pass through the spray zone, thereby recovering useful metal contained in the slag while treating the remaining slag.

It is other object of the present invention to convert the slag remaining after the recovery of useful metal, into a chemically more stable compound having resistance to weathering.

It is another object of the present invention to convert the slag remaining after the recovery of useful metal, into a chemically more stable spheroid which can be used in various applications .

It is also another object of the present invention to provide a method for recovering useful metal from slag while treating the slag, which can be performed in a more safe and effective manner by dividing the above spray zone into a first spray zone and a second spray zone .

It is still another object of the present invention to provide a method for recovering useful metal from slag while treating the slag, which can be performed in an efficient manner by maintaining a contact angle of a molten slag to an air stream blown from an air blower at a spray zone at an angle of about 95 °.

The above and other objects and aspects of the invention will be apparent from the following detailed description and embodiments.

The present invention comprises: spraying small water drops together with air at a constant blowing pressure to form a spray zone; dropping a molten state- converter slag having a desired temperature to the spray zone to rapidly cool the molten slag, thereby recovering useful metal of high purity from the slag while converting the remaining slag into a spheroid having a stable structure; and sorting the spheroid depending on the diameter and/or weight for reclamation. The spray zone may be an air stream already containing water vapor at a desired extent without being sprayed with separate water.

The converter slag of a molten state is rapidly cooled by the sprayed water, while being formed into particles (i.e., slag balls) by the blown air stream. In this way, iron components contained in the slag are separated from the slag. This is because only slag other than iron can be changed into another structure, i.e., a spinel structure. That is to say, the iron component and the slag other than the iron component are mostly physically separated during the cooling step with the blown air stream. Thus, the iron component heavy in weight is dropped underneath an air blower without being carried away with the blown air stream, whereas the remainder of the slag is carried away to a distance with the blown air stream and dropped to a point in a distance away from the point at which the iron component is dropped. In addition, some slag that is dropped together with the heavy iron component underneath the air blower without being carried away with the blown air stream is a material having a spinel structure different from that of the iron component. The slag dropped together with the iron component is readily separated from the iron component even when only a light degree of physical vibration is applied. As a result, this^' allows useful metal (iron) of high purity to be recovered.

Principles will now be described in detail that iron and slag are separated from each other by the above treatment and that the slag is converted into a weathering-resistant compound having a stable structure.

Recently, the converter slag generated from the steel-making process is generally composed of components listed in Table 1 below. Table 1: Chemical Components (% by weight) of Converter Slag

If the slag of these components is dumped itself in the sea, calcium oxide (CaO) among the slag components can then be converted into an alkaline compound such as Ca(OH)₂, etc. by seawater and salt to pollute seawater. Thus, this component is likely to be a serious obstacle to a habitation of fishes and sea plants.

Meanwhile, if the slag is left to stand on the ground, an alkaline fine powder such as calcium oxide, etc. can be scattered into air by a weathering process to injure the adjacent residential area and natural environment.

In the weathering process of the slag, a mechanical weathering (physical weathering) and a chemical weathering simultaneously occur while interacting. The mechanical weathering means that the slag is weathered by mechanical force. For example, as the air is heated and cooled, the slag undergoes repeated expansion and contraction resulting in very small crevices in which freezing, hydration and salt crystal extraction occur. The slag ultimately splits due to inner expansion into pieces, loses its crystal lime water, and finally becomes powder. In addition, the slag is broken down by the mechanical destruction of living things, such as microorganisms or plants. On the other hand, the chemical weathering means that the constituent of the slag is chemically changed and is decomposed by chemical action of chemicals or living things, hydration, hydrolysis, oxidation, reduction, carbonation, ion exchange and dissolution. The chemical weathering process is generally dependent on properties of the slag, pH of water, the amount of compound dissolved in water, such as oxygen, carbonic acid gas, ammonia, inorganic acid and organic acid, temperature and the like. By way of example, on the surface of the slag exist unsaturated radicals, which are generally reactive points with water, hydrolysis and hydration occurring thereat. Accordingly, metal ion M⁺ in the slag becomes MOH, while H⁺, if it exists in the slag, permits the ion of crystal to be changed into hydroxide ion (OH^~) which combines with CaO, MgO and the like to demolish the existing structure thereof. As a result, the slag is broken down into particles such as clay which is then dehydrated into dry, fine powder. Consequently, the slag is decomposed by these weathering processes into a powder of CaC0₃, CaO, Ca(OH)₂, MgO, Fe₂θ₃ or a mixture thereof, which then blows into the air and causes serious pollution problems .

However, in the method for recovering the useful metal and treating the slag according to the present invention, the slag melted in and discharged from the converter is passed through a water drop-containing blown air stream, in a state where the molten slag has a desired temperature. Thus, the slag is rapidly cooled, and the iron component of the slag is separated from other components of the slag. The iron heavy in weight is not carried out away with the blown air stream, and thereby allows the useful metal to be recovered. The remaining components of the slag are converted into spherical, stable, weathering-resistant material. Although some components of the slag in addition to the iron component are also dropped underneath the air blower, the slag components dropped together with the iron component are readily separated from the iron component by only a little degree of physical impact as the slag components have a spinel structure unlike the iron component. Meanwhile, the slag light in weight is carried away to a distance with the blown air stream in a spray zone and then dropped. The carried away distance of the slag varies depending on a diameter and/or weight of a slag ball (spheroid) .

The spherical slag balls, from which the useful metal are recovered and which are resistant to weathering, are sorted depending on their size selected according to their end-use using a sorting means . The sorted slag balls may be used in various applications such as a coagulant for wastewater, a shot ball, cement, an aggregate for concrete, a lightweight aggregate and the like. The sorting means may be a plurality of screens having different mesh sizes. In the case of sorting the slag balls using a weight difference therebetween, a group of slag balls having similar diameter may be dropped to a blown air stream to be sorted.

The reason why the slag treated according to the present invention has weathering resistance will now be described.

Generally, slag is said to be a non-metallic composition generated by the action of fluxes in making steel. The fluxes are added into a converter in order to improve the smelting effect. As a flux, there are mainly used lime, (containing CaO as a main component) , limestone (CaC0₃) , fluorite and the like. A coolant for the converter slag is mainly composed of Fe₂0₃ and CaO as shown in Table 1 above. In a melted state, CaO of strong basicity and Fe₂0₃ of strong acidity along with Si0₂ and MgO allow the slag to have a suitable silicate degree, so that the slag possesses a necessary viscosity.

In the case of slowly cooling the melted slag, the respective ions are solidified into their own constituent molecule (Fe₂0₃ and CaO) . These molecules come then to aggregate and are linked, respectively, so that the respective crystals are distributed in the solid slag. Fe₂0₃ and CaO so deposited are individually subjected to the weathering process without being formed into a mixture thereof. Thus, they are scattered in a powder state into the air. Especially, CaO plays an important role during the weathering.

However, when the slag is allowed to pass through the blown air stream to which water is sprayed as in the present invention, uniformly distributed ions such as Ca²⁺, O^2", Fe²⁺ and the like exist in a solid solution state while being solidified. As a result, the ions exist in the form of composite oxides such as CaO Fe₂0₃, Si0₂Fe0₃, MgO Fe₂0₃ and the like. These composites are composites of a bivalent oxide (CaO, Si0₂, or MgO) and a trivalent oxide (Fe₂0₃) that are a spinel type. These composites can be represented by the formula of X^X10- Y^I:π ₂θ₃ or (Me)^2~0^2~ ₄. The structure of such a new phase is a kind of structure of crystal represented by ABO compound wherein A and B are metal elements. This structure has a feature of regular octahedron. Namely, the structure is a general spinel structure which is subjected to a cubic system and in which the oxygen atoms consists of a cubic close-packed structure. A and B are placed in the spaces among the oxygen atoms of the cubic close-packed structure, A atom being in a tetrahedral hole consisting of 4 oxygen atoms and B atom being in an octahedral hole consisting of 6 oxygen atoms .

Spinel is originally said to be MgAl₂0₄, since the spinel structure is found first in this compound. Such crystals having the spinel structure are both abundant in nature as well as being made artificially. The spinel-type crystal is relatively hard with the hardness of about 5.5 to 8, shows a luster like glass and has a high reflective index. Of the crystals, those that are transparent in appearance without color are so hard that they may be used as jewels. In addition, the transparent crystals are weathering- and corrosion-resistant, and can become as weathering- and corrosion-resistant as ruby and crystallized quartz. In other words, chemical reactions, all of which are carried out by the motion of ions or electrons, do not proceed in the slag as the spinel-type composite oxides, constitute the surface of the slag. Accordingly, the slag with the present invention exhibits an improved resistance to chemical and physical deterioration such as weathering and hence is maintained at its original state. As a result, the slag converted into the spinel structure according to the present invention is in the form of a chemically stable spheroid and is no longer subjected to the weathering process. Thus, the slag treated according to the present invention does not cause a pollution problem caused by heavy metals effluent during the acidification of the conventional slag by the weathering process, and also may be used in various applications .

Meanwhile, in the practice of the present invention, the spray zone may be divided into a first spray zone and a second spray zone, if necessary. Namely, the present invention provides a method for recovering useful metal from a slag while treating the slag, the method comprising forming a single air stream flowing from a first spray zone to a second spray zone; dropping a molten slag to the air stream at the first spray zone so that the molten slag is rendered to be free of an iron component; spraying water to the molten slag at the first spray zone to cool the molten slag; and spraying water to the remaining slag at the second spray zone to further cool the remaining slag.

The remaining slag, from which the useful metal is recovered at the first spray zone and which is mostly composed of a spheroid, i.e., a granulated slag (slag ball) , flies with the air stream to the second spray zone. Although the remaining slag can seem to be granulated and converted into the spinel structure to a certain extent in the first spray zone, it is additionally cooled with the sprayed water in the second spray zone such that a portion of the slag that was not completely converted into the spinel structure or has an incomplete spinel structure is converted into a complete, additional spinel structure. Also, in the case of dividing the spray zone into the first and second spray zones, it has an effect in that the slag is allowed to cool in a safer operational manner. Namely, in the case of limiting the spray zone to only one station, a relatively large amount of water is sprayed to the slag of high temperature at a time. This can result in a somewhat hazardous working environment due to a sudden reaction of the hot slag and the large amount of water, and also can make a uniform cooling through a gradual cooling difficult. As a result, the spray zone is divided into two places and thus the cooling is carried out in two stages.

The foregoing has been mainly described regarding the case of dropping the molten slag to the blown air stream while spraying water to the molten slag, but the object of the present invention can be also accomplished by simply dropping the molten slag to the blown air stream without spraying separate water to the molten slag. Namely, the molten slag can be also converted into the spinel structure by water vapor contained in the atmosphere. However, in order to treat a large amount of slag simultaneously and to treat the produced slag ball having a low temperature at which the handling of the slag ball becomes easy, it is preferred to spray water as described in the following Best Mode for Carrying Out the Invention. Therefore, the dropping of the molten slag to a zone through which a vapor water-containing air flows must be understood to be also included in the scope of the present invention as described in the accompanying claims. Namely, the phrase "spraying water to a molten slag" as indicated in Claim 1 must be understood to also include the fact that the blown air stream itself contains water drop (water vapor) .

Brief Description of the Drawings Fig. 1 is an outlined view illustrating a method for recovering useful metal from and treating a slag according to the present invention; and

Fig. 2 is an actual size photograph of a slag ball treated according to a method of the present invention.

Best Mode for Carrying Out the Invention

Fig. 1 is a view schematically showing an embodiment of the present invention. Referring to Fig.l, a molten slag 3 discharged from a converter is transported to a turndish 2 from which the molten slag 3 is then dropped to a blown air stream 4. At this time, the molten slag is sprayed with water to be cooled. The molten slag is divided into iron and the remaining slag other than iron by the sprayed water and the blown air stream, and the remaining slag is granulized and flies with the blown air stream. In this case, the blowing pressure varies depending on the amount of the slag to be treated, the viscosity of the slag, etc. For instance, a blast pressure of about 800 m³/min x 700mmAg is used relative to 100 tons of slag.

The slag dropped to the blown air stream is rapidly cooled with the sprayed water (or water vapor contained in the blown air stream) , and components other than iron that are present in the molten slag are converted into composite oxides having a spinel structure. Thus, iron and the slag components other than iron are separated from each other. An iron component 5 heavy in weight is not carried away with the blown air stream and is dropped underneath an air blower 1. The other slag component light in weight is carried away to a distance with the blown air. The useful metal is different from the slag in specific gravity at about 7:15. The heavy slag (iron component) 5 dropped underneath the air blower can be divided into useful metal (iron) and a slag component by only a light vibration .

Meanwhile, the slag which was carried away to a distance from the air blower and converted into the spinel structure is sorted depending on their size selected according to their end-use by a sorting means.

^• For instance, in the case of sorting the slags according to their size, the slags can be sorted by means of a plurality of screens having different mesh sizes. More specifically, the slags can be sorted into those having various sizes such as 0.5 mm or less (for a wastewater coagulant), 0.5 to 3 mm (for shot ball or grit), 3 mm or more (for fine aggregate or high-grade decorative material) and the like according to a diameter of the slag balls. However, since the slag group which was sorted according to the size to have a similar diameter may also contain a slag group yet containing useful metal at a somewhat amount, the slag group may be dropped vertically into the blown air stream of a constant pressure such as 200 CCM x 800 mmAg to sort a a high- weight slag containing useful metal at a somewhat amount, if necessary. Namely, as the heavy slags containing a relatively large amount of useful metal are short in carried away distance and thus are dropped underneath the air blower, these can be collected, and sent to a furnace in which the useful metal is then recovered. The blown air stream may be sprayed with water in various manners. However, in an embodiment of the present invention, there is used a manner in that small water drops are sprayed to the slag from nozzles (not shown) disposed around a circumferential upper portion of the air blower 1.

The blown air stream 4 effluent from the outlet of the air blower is contacted by intersection with the molten slag 3 being dropped to the ground. With respect to the contact angle by intersection, it was confirmed from several experiments that, when the blown air stream 4 effluent from the air blower 1 was upwardly sprayed toward the turndish 2 at an angle of about 3 ° to 7° with respect to a horizontal plane, and preferably about 5 ° with respect to a horizontal plane, the slag balls were most effectively produced, and also separated well according to their size.

The slag passed through the spray zone is in a state where the separation from the iron component was achieved during the cooling process to a somewhat extent. The light slag, from which useful metal was separated, can seem to be converted into the spinel structure to a somewhat extent, but can not seem to be converted to a complete extent. For this reason, in order for the light slag, from which the useful metal was recovered, to be converted into a more complete spinel structure, we have carried out an experiment in which another zone (a second spray zone; not shown) being further sprayed with small water drops is formed at a second place, and the light slag carried away with the blown air stream is further contacted with water in the second spray zone. As a result of this experiment, we confirmed that the additional contact of the slag and water at the second spray zone stimulates the conversion of the slag into the spinel structure. Namely, the slags that were not changed to the spinel structure by the first contact with water were completely changed to the spinel structure by the second contact with water.

Depending on a steel mill, there can be a molten slag having a viscosity which is so high that the slag may not be applied to the slag treating process as described above. In this case, oxygen can be added into the blown air stream at a suitable amount corresponding to a viscosity of the slag so that the slag viscosity can be reduced by the reaction of oxygen with the slag. This allows the slag to be treated. The amount of oxygen present in the blown air stream, to which the suitable amount of oxygen was added, is in excess of the amount of oxygen present in the general atmosphere.

Meanwhile, the light slag containing the useful metals at a small amount has CaO as its main component, similarly to the powdery waste slag pulverized by the conventional method. However, the waste slag treated by the conventional method reacts with moisture or water present in the atmosphere to occur a conversion such as CaO + H₂ — > Ca(OH) _{2 r} so that it can not be kept for a lengthy period of time. On the contrary, the low-weight slag treated by the present invention is not powdery, and also has weather resistance. As a result, the low- weight slag is very useful in view of the resource reclamation . Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Industrial Applicability

As apparent from the above description, the method for recovering the useful metals from and treating the slag in accordance with the present invention allows the useful metals to be recovered at an inexpensive cost as it does not require an expensive equipment. Moreover, the method according to the present invention does not involve the pulverization of slag and thus eliminates the polluting factors such as noise and dust. Further, the present invention does not generate the powdery slag which is difficult to be discarded as a waste. Additionally, the present invention allows the low- weight slag remaining after the recovery of useful metals to be reclaimed in various applications.

Claims

1. A method for recovering useful metal from a slag generated in a steel-making process while treating the slag, the method comprising: dropping a molten slag to a horizontally blowing air stream while spraying water to the slag.

2. The method of Claim 1, in which the blowing air stream is upwardly sprayed to the molten slag at an angle of 3 ° to 7 ° with respect to a horizontal plane, and preferably at an angle of 5 ° with respect to a horizontal plane.

3. The method of Claim 1, in which the spraying of water to the molten slag is carried out in such a manner that water drops generated from a plurality of nozzles disposed around an upper circumferential portion of an outlet of the air blower, from which the air stream is blown, are entrained in the air stream.

4. The method of Claim 1, in which the air stream, into which oxygen was introduced, is sprayed into the molten slag. 5. A method for recovering useful metal from a slag generated in a steel-making process while treating the slag, the method comprising the steps of: forming a single air stream to be blown from a first spray zone to a second spray zone; dropping a molten slag to the air stream at the first spray zone in such a fashion that the dropped slag flies from the first spray zone to the second spray zone by virtue of the air stream while being rendered to be free of iron components at the first spray zone; spraying water to the molten slag at the first spray zone; and additionally spraying water to the remaining slag, free of the iron components, at the second spray zone.

6. The method of Claim 5, in which the blowing air stream at the first spray zone is upwardly sprayed to the molten slag at an angle of 3 ° to 7 ° with respect to a horizontal plane, and preferably at an angle of 5 ° with respect to a horizontal plane.

7. The method of Claim 5, in which the spraying of water to the molten slag at the first spray zone is carried out in such a manner that water drops generated from a plurality of nozzles disposed around an upper circumferential portion of an air blower at an outlet of the air blower, from which the air stream is blown, are entrained in the air stream.

8. The method of Claim 5, in which the air stream, into which oxygen was introduced, is sprayed into the molten slag at the first spray zone.

9. A method for recovering useful metal from a slag generated in a steel-making process while treating the slag, the method comprising: selecting slag balls having a diameter of 0.5 mm or less from the slag treated in accordance with a method as set forth in any one of Claims 1 to 8, so that the selected slag balls are used for a wastewater coagulant.

9. A method for recovering useful metal from a slag generated in a steel-making process while treating the slag, the method comprising: selecting slag balls having a diameter of 0.5 mm to 3 mm from the slag treated in accordance with a method as set forth in any one of Claims 1 to 8, so that the selected slag balls are used for a shot ball or a grit.

11. A method for recovering useful metal from a slag generated in a steel-making process while treating the slag, the method comprising: selecting slag balls having a diameter of 3 mm or more from the slag treated in accordance with a method as set forth in any one of Claims 1 to 8 , so that the selected slag balls are used for a fine aggregate or a high-grade decorative material .