WO2007145481A1

WO2007145481A1 - Method of cooling slag generated during preliminary treatment of molten iron

Info

Publication number: WO2007145481A1
Application number: PCT/KR2007/002905
Authority: WO
Inventors: Tae Ho Kim; Hyung Suk Oh; Jeong Beom Seo
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2006-06-15
Filing date: 2007-06-15
Publication date: 2007-12-21
Anticipated expiration: 2008-12-15
Also published as: KR100775503B1; CN101466850A; CN101466850B; JP2009540127A; JP4865858B2

Abstract

The present invention relates to a method of cooling slag generated during the preliminary treatment of molten iron, which is commonly executed to remove sulfur and the like contained in the molten iron. The method includes cooling slag in the preliminary treatment of molten iron, including coating slag in a molten phase on the molten iron pretreatment slag, which is piled up, prior to water-cooling the molten iron pretreatment slag, wherein the slag in a molten phase contains 10 wt% free-CaO or less, and essential active oxygen. Thanks to this cooling method, it is possible to cool molten iron pretreatment slag to the ambient temperature at a cooling rate that is accelerated by 50 % or more, compared to the conventional method. Thus, it is economical.

Description

METHOD OF COOLING SLAG GENERATED DURING PRELIMINARY TREATMENT OF MOLTEN IRON

Technical Field

[1] The present invention relates to a method of cooling slag, and more particularly to a method of rapidly cooling slag (hereinafter, simply referred to as "molten iron pre- treatment slag") generated during preliminary treatment of molten iron, at low cost. The cooling rate of the molten iron pre treatment slag is generally very slow.

[2] The term "preliminary treatment" or "preliminary process" according to the present invention refers to desulfurization treatment and the like of a molten pig iron. Background Art

[3] In the steel industry, various types of slag are produced as by-products, with different compositions and properties depending upon kinds of processes and facilities, and also upon kinds of steel produced by melting.

[4] For example, blast furnace slag is produced as by-product from a blast furnace which is used in a process for producing pig iron. Furthermore, molten iron pretreatment slag, converter slag, and electric furnace slag are respectively produced as by-products from a molten-iron pretreatment facility, a converter, and an electric furnace.

[5] Generally, slag is at a temperature of 1000 ⁰C or higher. Hence, slag is typically cooled at a site near the furnace prior to being transported for recovery and recycling of metal (base metal) contained therein. Such cooling is typically conducted using water cooling, in order to reduce the treatment area and the cooling-down period.

[6] Also, the molten iron pretreatment slag is separated from molten iron and piled up at the designated site adjacent to the blast furnace, and then water-cooled. Disclosure of Invention Technical Problem

[7] However, the conventional water-cooling methods have problems in that, since the time period necessary to cool the molten iron pretreatment slag all the way to the center thereof is considerably long compared to other types of slag, an excessively high amount of water must be consumed during water-cooling process.

[8] Although the cooling rate can be improved by spreading the slag over a large area rather than piling up the slag, this process requires a considerable area.

[9] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a method of completely cooling molten iron pretreatment slag to the center thereof, with a cooling rate that is equal to or higher than the cooling rate achieved with other iron-making slag.

[10] Another object of the present invention is to provide a method of cooling slag in the preliminary treatment of molten iron in an economical manner. Technical Solution

[11] In order to accomplish the above objects, the present invention provides a method of cooling slag in the preliminary treatment of molten iron, including applying slag in a molten phase on the molten iron pretreatment slag which is piled up, prior to water- cooling the molten iron pretreatment slag, wherein the slag in the molten phase contains 10 wt% free-CaO or less, based on the total weight, and active oxygen.

[12] In this method, the active oxygen may be present in the form of gaseous oxygen and/ or ferric oxide such as FeO or Fe O contained in the molten slag, and the molten slag applied on the molten iron pretreatment slag is preferably converter slag. The thickness to which the molten slag is applied is preferably in the range of 50-300 mm, regardless of the height to which the molten iron pretreatment slag is piled.

Advantageous Effects

[13] According to the method of cooling slag in the preliminary treatment of molten iron, which is designed in the above-described manner, it is possible to cool molten iron pretreatment slag to the ambient temperature at a cooling rate that is accelerated by 50% or more, compared to the conventional method.

[14] In addition, since the method according to the present invention can utilize converter slag, which is concomitantly generated during the process of making molten steel, it is economical. Best Mode for Carrying Out the Invention

[15] The method of cooling molten iron pretreatment slag has been derived from the following recognized facts.

[16] The first fact is that the carbon component contained in molten iron pretreatment slag hinders the cooling of the molten iron pretreatment slag. More specifically, the carbon component in molten iron pretreatment slag is precipitated on the surface of the molten iron pretreatment slag in the course of cooling. Since the precipitated carbon has poor wettability with water, the precipitated carbon hinders the infiltration of the water into the molten iron pretreatment slag.

[17] The second fact is that quicklime (about 15% or more), which does not react to components such as sulfur, contained in a large amount of quicklime, which is used in the preliminary treatment of molten iron and thus remains in the slag, hinders the cooling of molten iron pretreatment slag. More specifically, the remaining quicklime (CaO) reacts to atmospheric moisture (CaO + H O → Ca(OH) ), and expands (about double), thus causing the slag to be fractured/powdered. The slag powder not only has small clearances therebetween but also agglomerates (forms a slurry) when water is sprinkled thereon, thus preventing water from infiltrating into the molten iron pre- treatment slag. Meanwhile, the hydration of quicklime proceeds exothermically, and the reaction heat generated in the exothermic process evaporates some of the cooling water sprinkled on the slag. The evaporation of cooling water increases the required amount of cooling water.

[18] For this reason, the cooling water does not infiltrate into the heaped molten iron pre- treatment slag, but flows down along the surface of the slag, thereby retarding the cooling rate of the molten iron pretreatment slag. In addition, the required amount of cooling water is excessively increased.

[19] Hence, in order to remove fine powder generated by the hydration of carbon and free-CaO, which hinders the infiltration of cooling water, from the surface of the molten iron pretreatment slag, the method according to the present invention applies molten slag, in particular, converter slag (hereinafter, the slag applied on the molten iron pretreatment slag is simply referred to as "coating slag"), which contains 10 wt% or less of free-CaO, based on the total weight thereof, and essential active oxygen, on the molten iron pretreatment slag.

[20] In this method, if the coating slag contains free-CaO in an amount greater than 10 wt%, or the coating slag is in a solid phase rather than in a molten phase, the effect of promoting the cooling of molten iron pretreatment slag is negligible. That is, it is believed that if the coating slag is not in the molten phase, the slag, which is powdered by the hydration of quicklime, is not sufficiently removed from the surface of the molten iron pretreatment slag, and, if the coating slag contains free-CaO in an amount greater than 10 wt%, the function of the coating slag, that of hindering the molten iron pretreatment slag from being powdered, is decreased or lost.

[21] Furthermore, the slag applied on molten iron pretreatment slag forms a porous structure, which will be described below, thus enabling cooling water to easily infiltrate into the molten iron pretreatment slag.

[22] That is, active oxygen (most of the active oxygen is probably present in the form of ferric oxide), contained in the coating slag, reacts to carbon precipitated on the surface of the molten iron pretreatment slag, and thus removes the carbon (the carbon is removed in the form of CO). Then, gas (such as CO), generated by reaction with the carbon, escapes upward through the coating slag layer, thus forming the coating slag layer into a porous structure. At this point, the porous structure must not have separate voids formed in the inside of the coating slag layer, and must have channels (hereinafter, referred to as "permeable channels), which are connected between the inside and the surface of the coating slag layer. Thus, cooling water sprinkled on the coating slag layer can easily infiltrate into the molten iron pretreatment slag from the surface of the coating slag layer through the permeable channels.

[23] There is a question of how much oxygen must be contained in the coating slag in order to realize a porous structure. However, since the amount of oxygen contained in the coating slag may vary depending on the particular operation, it is not easy to quantify the required amount of oxygen. In most cases, when the coating slag contains oxygen in an amount of 1 wt% or more, it is believed that the effects intended by the present invention will be sufficiently obtained.

[24] According to the present invention, the slag, which is most suitable for satisfying the conditions of the coating slag, is converter slag, which is concomitantly generated in the course of operation of a converter. The converter slag contains oxygen in an amount of 1 wt% or more, and free-CaO is present in an amount of 5 wt% or less, based on the weight of slag.

[25] The thickness of the coating slag, which is applied on the molten iron pretreatment slag, must be in the range of 50-300 mm. If the thickness of the coating slag is less than the above range, the effect of eliminating oxygen is negligible, thus precluding the realization of a porous surface. Meanwhile, if the thickness of the coating slag is greater than the above range, air bubbles cannot escape to the outside, but are trapped in the coating slag layer, thus precluding the realization of a porous coating slag layer.

[26] Here, the height to which the molten iron pretreatment slag, on which slag is applied, is piled does not play an important role. Regardless of whether or not the height to which the molten iron pretreatment slag is piled is high, where molten iron pretreatment slag is treated on the surface thereof with the coating slag according to the present invention, cooling water can quickly infiltrate into the molten iron pretreatment slag. The reason why the molten iron pretreatment slag cannot infiltrate into the molten iron pretreatment slag is that fine powder, which is generated by reaction of carbon and free-CaO on the surface of the molten iron pretreatment slag, impedes the infiltration of water into the molten iron pretreatment slag. Consequently, when the blocking layer, such as the fine powder on the surface, is eliminated, the molten iron pretreatment slag can be easily infiltrated with the cooling water through the permeable channels. Mode for the Invention

[27] In order to determine the conditions for the coating slag, molten iron, obtained in a blast furnace operation, was desulfurized, and slag was removed from the molten iron and was then heaped. Subsequently, the heaped molten iron pretreatment slag was evenly applied with coating slag, and was repeatedly subjected to sprinkling and cooling procedures.

[28] The principal point to be checked in this determination was to observe influences exerted on molten iron pretreatment slag by 1) the amount of free-CaO contained in coating slag, 2) the presence of active oxygen, and 3) the phase of the coating slag.

[29] Table 1 below shows representative results of time periods for which it is required to cool molten iron pretreatment slag to a temperature of 100 ⁰C after the molten iron pretreatment slag is applied with coating slags having the above three different conditions to a thickness of 250 mm. Here, the coating slag used in Comparative Examples was dephosphorized slag and cast slag, and the coating slags used in Examples were converter slags. Example 1 shows a result obtained by cooling molten iron pretreatment slag by directly sprinkling water without the use of coating slag.

[30] Table 1 [Table 1] [Table ]

[31] As is apparent from Table 1, the time period required to cool molten iron pretreatment slag was 30hr or more when coating slag was not used. [32] When coating slag in a molten phase was applied on molten iron pretreatment slag, the cooling time required for the molten iron pretreatment slag was considerably shortened. As seen in Comparative Examples 5 and 6, in order to shorten the cooling time of the molten iron pretreatment slag through the modification of a surface layer of the molten iron pretreatment slag, active oxygen must be present in coating slag, and the coating slag must be in a liquid phase.

[33] Free-CaO in the coating slag, when present in an amount of 10 wt% or less, effectively cooled molten iron pretreatment slag (the cooling time was shortened by about 50%), and free-CaO in the coating slag, when present in an amount of 5 wt% or less, even more effectively cooled molten iron pretreatment slag. When free-CaO was present in an amount greater than 10 wt%, although the cooling time was somewhat shortened, a considerably long cooling time of 20 hours of more was still required.

[34] Furthermore, in order to observe the influences exerted on the water-cooling of molten iron pretreatment slag by the thickness of the coating slag applied on the molten iron pretreatment slag and the height of the piled molten iron pretreatment slag, an experiment was conducted as follows.

[35] Specifically, the slag having the condition of Comparative Example 1 in Table 1, that is, converter slag containing free-CaO of 5-10 wt% and oxygen of 1 wt% or more, was applied on molten iron pretreatment slag, and was then sprinkled with water. At this point, the time period required to cool the molten iron pretreatment slag to 100 ⁰C was measured. Thereafter, the cooling time was measured while varying the coating thickness of the converter slag and the heaping height of the molten iron pretreatment slag. The exemplary results are given in Table 2 below.

[36] Table 2 [Table 2] [Table ]

[37] As is apparent from Table 2, the influence of the height of molten iron pretreatment slag on the cooling rate was negligible, compared to the influence of the thickness to which coating slag was applied. This result can be seen by comparing Example 1 with Example 3. Specifically, even when the heaping height of the molten iron pretreatment slag is increased to 1000 mm from 300 mm, if the coating thickness of the coating slag is maintained in the range of 100-300mm, there is only about 1 hour difference in the cooling time of the molten iron pretreatment slag. It is believed that the difference of 1 hour naturally results from the fact that the amount of molten iron pretreatment slag to be cooled is increased in proportion to the increase in the piling height of the molten iron pretreatment slag. From the above results, it is noted that the cooling water infiltrates the molten iron pretreatment slag at a considerably quick rate due to the surface modification of the molten iron pretreatment slag according to the present invention.

[38] Meanwhile, when the coating thickness of the coating slag was unduly decreased

(less than 50 mm) or increased (larger than 300 mm), the cooling efficiency was greatly lowered, and bore no relation to the height to which the molten iron pretreatment slag was piled. This result signifies that when the coating thickness of the coating slag is 50-300 mm, and preferably 100-300 mm, permeable channels are efficiently generated.

Industrial Applicability

[39] The present invention can be applied to iron-making factories. In particular, since the present invention can cool slag generated during the preliminary treatment of molten iron at a cooling rate higher than that of the conventional method, the present invention contributes to cost reduction in an iron-making processes.

Claims

[1] A method of cooling slag generated during preliminary treatment of molten iron, comprising: piling up the molten iron pretreatment slag; coating the piled molten iron pretreatment slag with slag that is concomitantly generated in an iron-making process; and cooling the molten iron pretreatment slag using water; wherein the coating slag is in a molten phase containing 10 wt% or less of free-

CaO, and wherein the coating slag contains active oxygen which forms a porous structure at least in the coating slag using air bubbles generated in a reaction with carbon present on a surface of the molten iron pretreatment slag, thereby allowing water to infiltrate into the molten iron pretreatment slag. [2] The method according to claim 1, wherein the coating slag contains about 1 wt% or more of the active oxygen. [3] The method according to claim 1 or 2, wherein the coating slag is applied on the molten iron pretreatment slag to a thickness of 50-300 mm. [4] The method according to claim 1, wherein the coating slag is converter slag, which is concomitantly generated in a converter operation. [5] A method of cooling slag generated during preliminary treatment of molten iron, comprising: piling up the molten iron pretreatment slag; coating the piled molten iron pretreatment slag with slag in a molten phase, which is concomitantly generated in a converter operation, to a thickness of

50-300 mm; and sprinkling cooling water on the molten iron pretreatment slag coated with the converter slag. [6] The method according to claim 5, wherein the converter slag contains 10 wt% or less of free-CaO, based on a total weight thereof. [7] The method according to claim 5 or 6, wherein the converter slag contains 1 wt% or more of active oxygen based on the total weight.