Ferro-nickel slag

Abstract

This technology provides a technique for suppressing the elution of hexavalent chromium into water from ferronickel slag containing chromium, which is a by-product produced when manufacturing ferronickel alloys in a rotary kiln. [Solution] The chemical components, on a dry mass basis, are SiO2: 30-70%, MgO: 10-40%, CaO: 1.0-10%, Al2O3: 1.0-5.0%, FeO: 1.0-20%, Cr2O3: 0.1-5%, S: 0.01-0.3%, C: 0.01-3%, Ni: 0.50% or less, and unavoidable components. The slag basicity, expressed as the mass ratio of MgO to SiO2, is in the range of 0.30-0.80, and the ferronickel slag is composed of slag particles containing a (Mg,Fe)(Cr,Al)2O4 chromite phase and unburned carbon particles.

Description

[Technical Field]

[0001] The present invention relates to ferronickel slag containing chromium, and in particular to ferronickel slag that suppresses the elution of hexavalent chromium into water and is suitable for use as a civil engineering slag material in compliance with soil environmental standards. In this specification, "x to y" representing a range of values ​​means "x or more and y or less" and includes the boundary value. The unit of mass "t" represents 1000 kg. The unit of volume "L" represents 10 -3 m 3 In this specification, the katakana notation "chromium" is a general term for elements contained in metal or compound form. The element symbol notation "Cr" is used in chemical formulas that represent the composition of compounds. [Background technology]

[0002] Ferronickel alloys are widely used as a nickel source for nickel-containing austenitic stainless steels, duplex stainless steels, alloy steels, etc. These ferronickel alloys have traditionally been produced by reducing nickel oxide and iron oxide contained in nickel oxide ore.

[0003] Examples of methods for producing ferronickel alloys include the following: Patent Document 1 discloses a method of drying and reducing using a rotary kiln; Patent Document 2 discloses a method of drying and reducing using a combination of a rotary dryer and an electric furnace; and Patent Document 3 discloses a method of reducing using an electric reduction furnace.

[0004] In the production of ferronickel alloys, ferronickel slag is generally generated when raw materials are melted and reduced to obtain ferronickel. This slag contains oxides such as SiO2, MgO, and Al2O3 from the ore. It also contains ferronickel fine sand and unburned charcoal, which cannot be separated using a rotary kiln production method. As shown in Patent Document 3, this ferronickel slag is used as a construction material, and it has been shown that heavy metals with low boiling points, such as cadmium and selenium, in the slag are volatilized, so there is no risk of leaching. However, trace amounts of chromium oxide contained in the ore are also contained in the slag. Chromium does not volatilize due to its high boiling point, and some of the chromium oxide may convert to hexavalent chromium depending on the conditions during discharge from the furnace or cooling, which has been a concern regarding the recycling of ferronickel slag.

[0005] Patent Document 4 discloses a method of using an air-blocking agent to prevent chromium in the slag from being oxidized to hexavalent chromium during cooling, while Patent Document 5 proposes a method of cooling in a controlled non-oxidizing atmosphere.

[0006] Patent Document 6 shows that, taking into consideration the production load, a maximum particle size was found that does not increase the amount of hexavalent chromium even when crushed after washing with water. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 05-295469 [Patent Document 2] Japanese Patent Application Laid-Open No. 2024-106954 [Patent Document 3] Patent Publication No. 2021-21040 [Patent Document 4] Japanese Patent Application Publication No. 2017-222917 [Patent Document 5] Japanese Patent Application Laid-Open No. 2014-237556 [Patent Document 6] Patent Publication No. 2021-112690 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the above-mentioned conventional techniques have the following problems. In other words, the technologies disclosed in Patent Documents 4 and 5 were impractical because they required a great deal of cost to implement. Furthermore, the technologies disclosed in Patent Documents 4 and 5 are primarily technologies for suppressing hexavalent chromium in steelmaking slag mainly composed of SiO2-CaO. These technologies are not specifically referred to as technologies for suppressing hexavalent chromium in ferronickel slag mainly composed of SiO2-MgO. The technology disclosed in Patent Document 6 does not guarantee compliance with the environmental standards set forth in Notification No. 46 of the Environment Agency. From the perspective of reusing ferronickel slag, optimizing slag size alone was insufficient to ensure that the amount of hexavalent chromium leaching entirely fell within the environmental standards set forth in Notification No. 46 of the Environment Agency.

[0009] Therefore, an object of the present invention is to provide a technology for suppressing the elution of hexavalent chromium into water from ferronickel slag containing chromium that is by-produced when a ferronickel alloy is produced in a rotary kiln. [Means for solving the problem]

[0010] The inventors recognized that the greatest challenge in recycling ferronickel slag is the further oxidation of some of the trivalent chromium oxide contained in the slag to hexavalent chromium oxide. Based on this understanding, they conducted extensive research to solve this problem. As a result, they discovered that trivalent chromium oxide is oxidized to hexavalent chromium oxide by reaction with oxygen in the atmosphere when the slag is discharged from a rotary kiln. They also found that the amount of hexavalent chromium oxide is significantly affected by the mineral phases contained in the slag. The present invention was developed based on this finding. The inventors decided to specify the composition of the slag (hereinafter referred to as ferronickel slag, or simply as slag) generated during ferronickel smelting in a rotary kiln and limit the main mineral phases contained therein. To this end, they discovered that by performing semi-smelting reduction before discharging the slag from the rotary kiln, ferronickel slag that can be used as a civil engineering slag material can be obtained at low cost, with a low amount of hexavalent chromium leaching. Based on these findings, the present invention was developed.

[0011] The ferronickel slag according to the present invention, which advantageously solves the above problems, has chemical compositions, on a dry mass basis, of 30-70% SiO2, 10-40% MgO, 1.0-10% CaO, 1.0-5.0% Al2O3, 1.0-20% FeO, 0.1-5% Cr2O3, 0.01-0.3% S, 0.01-3% C, 0.50% or less Ni, and unavoidable components, and is characterized by having a slag basicity, expressed as the mass ratio of MgO to SiO2, in the range of 0.30-0.80, and being composed of slag particles containing a (Mg,Fe)(Cr,Al)2O4 chromite phase and unburned carbon particles.

[0012] The ferronickel slag according to the present invention is (a) the slag particles further contain, as a main phase, a silicate mineral phase consisting of three phases: a CaO-SiO-AlO-MgO-FeO amorphous phase, a (Mg,Fe)SiO pyroxene phase, and a (Mg,Fe)SiO olivine phase; (b) the slag is composed of coarse-grained ferronickel slag and fine-grained ferronickel slag, the coarse-grained ferronickel slag having a mass fraction passing through a sieve with a nominal mesh size of 1.18 mm of 80% by mass or more and a mass fraction passing through a sieve with a nominal mesh size of 0.15 mm of 30% by mass or less, and the fine-grained ferronickel slag having a mass fraction passing through a sieve with a nominal mesh size of 0.075 mm of 60% by mass or more; This would be a more preferable solution to the problem.

[0013] The above-mentioned ferronickel slag is slag particles obtained as a residue when clinker consisting of an aggregate of ferronickel and slag is crushed and the ferronickel particles are separated and recovered by underwater gravity separation. The nominal mesh size of the sieve is based on the specifications of JIS Z8801-1:2019. [Effects of the Invention]

[0014] According to the present invention, even in the case of a slag composition containing chromium, the chromium is prevented from agglomerating into a phase that is easily oxidized upon atmospheric cooling, and therefore the amount of hexavalent chromium leaching can be suppressed to 0.05 mg / L or less in accordance with Environment Agency Notification No. 46 at low cost. Furthermore, while it has become difficult to obtain natural sand in recent years due to environmental conservation considerations, the present invention makes it possible to utilize by-products of ferronickel production, thereby making a significant contribution to global environmental conservation. Coarse-grained ferronickel slag can be used for asphalt aggregate, etc., and fine-grained ferronickel slag can be used for base materials, etc., and are therefore industrially useful. DETAILED DESCRIPTION OF THE INVENTION

[0015] The ferronickel slag according to one embodiment of the present invention is a slag suitable for civil engineering applications, with a low hexavalent chromium elution amount, as determined by the chemical composition and mineral phase. Hereinafter, the chemical composition will be expressed on a dry mass basis unless otherwise specified.

[0016] The chemical composition of the ferronickel slag according to this embodiment is 30-70% SiO2, 10-40% MgO, 1.0-10% CaO, 1.0-5.0% Al2O3, 1.0-20% FeO, 0.1-5% Cr2O3, 0.01-0.3% S, 0.01-3% C, 0.50% or less Ni, and other unavoidable components. The slag basicity, expressed as the mass ratio of MgO to SiO2, is in the range of 0.30-0.80. The mineral phase is composed of slag particles containing a (Mg,Fe)(Cr,Al)2O4 chromite phase and unburned carbon particles. The mineral phase of the slag particles preferably further contains, as a main phase, a silicate mineral phase consisting of three phases: a CaO-SiO-AlO-MgO-FeO amorphous phase, a (Mg,Fe)SiO pyroxene phase, and a (Mg,Fe)SiO olivine phase.

[0017] The (Mg,Fe)(Cr,Al)2O4 chromite phase is the most effective phase for maintaining trivalent chromium. By incorporating trivalent chromium, the (Mg,Fe)(Cr,Al)2O4 chromite phase can prevent trivalent chromium from being oxidized to hexavalent chromium when the ferronickel slag is cooled.

[0018] If the (Mg,Fe)(Cr,Al)2O4 chromite phase is not present, the chromium in the slag is incorporated into other mineral phases. Chromium incorporated into other mineral phases, especially the CaO-SiO2-Al2O3-MgO-FeO amorphous phase, is easily oxidized to hexavalent chromium. This can result in the amount of hexavalent chromium leaching exceeding the standard limit.

[0019] The CaO-SiO2-Al2O3-MgO-FeO amorphous phase was a liquid phase at high temperatures in a rotary kiln. The (Mg,Fe)SiO3 pyroxene phase and the (Mg,Fe)2SiO4 olivine phase were solid phases that crystallized at high temperatures in a rotary kiln. In an environment where these solid and liquid phases coexist, the raw materials are tumbled in the rotary kiln to efficiently reduce iron and nickel and grow ferronickel particles. Insufficient tumbling of the raw materials in the rotary kiln can reduce the reduction efficiency of iron and nickel, resulting in high production costs and making the process unprofitable. Furthermore, the ferronickel particles can become too fine, resulting in a large amount of ferronickel remaining in the slag without being completely separated and removed. Ferronickel also contains trace amounts of chromium, which can be oxidized to form hexavalent chromium when discharged from the rotary kiln.

[0020] In the section 5 to 10 meters from the rotary kiln discharge side, the upper end of the fired material rises from the vertical bottom end to a height on the circumference within a range of 60° to 120° in terms of the central angle of rotation. After that, the fired material at the upper end separates from the mass of fired material and rolls along the top surface of the mass, falling to the lower end of the fired material. This type of movement is defined as rolling. On the other hand, if the upper end of the fired material only rises from the vertical bottom end to a height on the circumference within a range of 60° in terms of the central angle of rotation, the fired material at the upper end does not separate from the mass of fired material and does not appear to fall to the lower end of the fired material. This state is defined as no rolling. The direction of rotation of the rotary kiln is not important.

[0021] <Chemical composition of ferronickel slag> The chemical components of the ferronickel slag of this embodiment will be described. Slag basicity: 0.30~0.80 Slag basicity is expressed as the mass ratio of MgO to SiO2. Slag basicity is an index that determines the rolling property of the fired material in a rotary kiln during ferronickel smelting. If the slag basicity exceeds 0.80 or falls below 0.30, the melting point becomes too high. Therefore, even if CaO or Al2O3 is contained, a liquid phase is not easily generated in the rotary kiln, and good rolling of the fired material is not achieved. When the slag basicity is in the range of 0.30 to 0.80, adding an appropriate amount of CaO or Al2O3 generates a liquid phase, allowing good rolling of the fired material. Therefore, the slag basicity is specified to be in the range of 0.30 to 0.80. Preferably, the slag basicity is in the range of 0.40 to 0.70.

[0022] SiO2: 30-70% SiO2 is one of the main components that determines the rolling properties of the fired material in a rotary kiln during ferronickel smelting. If the SiO2 concentration exceeds 70%, the slag basicity becomes too low. Therefore, even if CaO or Al2O3 is added, it becomes difficult to form a liquid phase in the rotary kiln, and good rolling of the fired material cannot be achieved. Furthermore, if the SiO2 concentration is below 30%, the reactivity of the fired material decreases, preventing the formation of the (Mg,Fe)(Cr,Al)2O4 chromite phase. Furthermore, chromium cannot be maintained in a trivalent state, resulting in an increase in the amount of hexavalent chromium produced when the material is discharged from the rotary kiln. For this reason, the SiO2 concentration is specified to be in the range of 30 to 70%. Preferably, the SiO2 concentration is in the range of 40 to 60%.

[0023] MgO: 10-40% MgO is also one of the main components that determines the rolling properties of the fired product in a rotary kiln during ferronickel smelting. Furthermore, MgO is an important component for forming a (Mg,Fe)(Cr,Al)2O4 chromite phase in ferronickel slag, thereby immobilizing chromium in a trivalent state. If the MgO concentration is less than 10%, the (Mg,Fe)(Cr,Al)2O4 chromite phase cannot be formed. If the MgO concentration exceeds 40%, the slag basicity becomes too high. Furthermore, even if CaO or Al2O3 is added, it becomes difficult to form a liquid phase in the rotary kiln, preventing good rolling of the fired product. Therefore, the MgO concentration is specified to be in the range of 10 to 40%. Preferably, the MgO concentration is in the range of 20 to 35%.

[0024] CaO: 1.0 to 10% CaO is a component necessary for generating a liquid phase in a rotary kiln during ferronickel smelting and for providing rolling motion to the fired product. If the CaO concentration is less than 1.0%, the liquid phase is too small and good rolling motion of the fired product cannot be obtained. If the CaO concentration exceeds 10%, the liquid phase ratio increases too much and rolling motion of the fired product cannot be obtained. Therefore, the CaO concentration is specified to be in the range of 1.0 to 10%. Preferably, the CaO concentration is in the range of 1.5 to 8%.

[0025] Al2O3: 1.0~5.0% Al2O3 is also a necessary component for generating a liquid phase in the rotary kiln during ferronickel smelting and providing rolling motion to the fired product. Al2O3 is also an important component for forming the (Mg,Fe)(Cr,Al)2O4 chromite phase and immobilizing chromium in trivalent form. If the Al2O3 concentration is less than 1.0%, the liquid phase is insufficient, preventing good rolling motion of the fired product. Furthermore, the (Mg,Fe)(Cr,Al)2O4 chromite phase becomes significantly reduced. If the Al2O3 concentration exceeds 5.0%, the liquid phase ratio increases too much, preventing good rolling motion of the fired product. Therefore, the Al2O3 concentration is specified to be in the range of 1.0 to 5.0%. Preferably, the Al2O3 concentration is in the range of 2 to 4%.

[0026] FeO: 1.0 to 20% FeO is an important component for forming the (Mg,Fe)(Cr,Al)2O4 chromite phase and fixing chromium in trivalent form. If the FeO concentration is less than 1.0%, the amount of the (Mg,Fe)(Cr,Al)2O4 chromite phase is significantly reduced. If the FeO concentration exceeds 20%, it indicates that the rotary kiln is in an oxidizing atmosphere. This may result in chromium being oxidized to hexavalent chromium. Therefore, the FeO concentration is specified to be in the range of 1.0 to 20%. Preferably, the FeO concentration is in the range of 2 to 18%.

[0027] Cr2O3: 0.1~5% The Cr2O3 content is preferably as low as possible. If the Cr2O3 concentration exceeds 5%, the excess chromium, exceeding the amount that can be fixed by the (Mg,Fe)(Cr,Al)2O4 chromite phase, is incorporated into other phases and partially oxidized to hexavalent chromium. This increases the amount of hexavalent chromium, potentially exceeding the environmental standard for hexavalent chromium leaching in accordance with Notification No. 46 of the Environment Agency. A Cr2O3 concentration below 0.1% means that excessive carbonaceous materials are used to reduce chromium to metallic chromium, which increases production costs. Therefore, the Cr2O3 concentration is specified to be in the range of 0.1 to 5%. Preferably, the Cr2O3 concentration is in the range of 0.5 to 3%.

[0028] S: 0.01 to 0.3% During ferronickel smelting, sulfur distributes in ferronickel alloys to form low-melting-point phases and promote the growth of ferronickel particles. If some sulfur remains in the ferronickel slag and the sulfur concentration falls below 0.01%, the ferronickel particles become finer, and the ferronickel particles cannot be completely separated and remain in the slag. Ferronickel also contains trace amounts of chromium, which oxidizes to form hexavalent chromium. If the sulfur concentration exceeds 0.3%, the sulfur content in the raw material is very high, and the sulfur concentration in the ferronickel also increases. When stainless steel and special steel are produced from ferronickel alloys, sulfur impairs the weldability of the alloy and reduces its hot workability. Therefore, the sulfur content in the alloy must be reduced, which increases the load on the refining process using AOD and VOD. Therefore, the sulfur concentration is specified to be in the range of 0.01 to 0.3%. Preferably, the sulfur concentration is in the range of 0.03 to 0.25%.

[0029] C: 0.01 to 3% Carbon is an essential element for reducing iron and nickel in ferronickel smelting. When the carbon concentration exceeds 3%, the reduction is so strong that FeO is excessively reduced. This prevents the formation of the necessary (Mg,Fe)(Cr,Al)2O4 chromite phase, resulting in excessive chromium in the liquid phase. This results in the chromium being oxidized during discharge from the rotary kiln, resulting in the formation of large amounts of hexavalent chromium. When the carbon concentration falls below 0.01%, this indicates a lack of carbon necessary for reduction, significantly reducing the efficiency of ferronickel smelting. Therefore, the carbon concentration is limited to a range of 0.01 to 3%.

[0030] Ni: 0.50% or less As mentioned above, the Ni concentration in the slag increases due to a decrease in reduction efficiency caused by a lack of good rolling, the refinement of ferronickel particles caused by S in the slag being less than 0.01%, or insufficient reduction caused by C in the slag being less than 0.01%. A Ni concentration above 0.50% indicates insufficient recovery of ferronickel. In other words, this significantly reduces the efficiency of ferronickel smelting and increases production costs. For this reason, the Ni concentration is specified as 0.50% or less.

[0031] Inevitable ingredients Components of ferronickel slag other than those mentioned above are inevitable components contained in the raw material nickel ore and recycled Ni raw materials, and inevitable components contained in or mixed into auxiliary materials added during the ferronickel smelting process, such as phosphorus compounds, alkali metal oxides, and alkaline earth metal oxides.

[0032] Contains unburned carbon particles Carbon particles react with oxygen (O2) in the atmosphere inside the rotary kiln as follows: 2C+O2→2CO↑ This reaction occurs preferentially over the chromium in the ferronickel slag, preventing the formation of hexavalent chromium. The absence of unburned carbon particles means that the oxygen (O2) in the atmosphere inside the rotary kiln reacts with the chromium in the ferronickel slag, which can produce hexavalent chromium. Therefore, the inclusion of unburned carbon particles is specified. It is preferable that all of the carbon contained is unburned carbon particles.

[0033] <Slag particle size> The slag preferably comprises coarse-grained ferronickel slag and fine-grained ferronickel slag. The coarse-grained ferronickel slag has a mass fraction passing through a sieve with a nominal mesh size of 1.18 mm of 80% by mass or more, and a mass fraction passing through a sieve with a nominal mesh size of 0.15 mm of 30% by mass or less. The fine-grained ferronickel slag has a mass fraction passing through a sieve with a nominal mesh size of 0.075 mm of 60% by mass or more. The nominal mesh sizes of the sieves are based on the specifications of JIS Z8801-1:2019. These particle sizes can be used as various slag civil engineering materials. [Example]

[0034] The effects of the present invention will be explained below using examples and comparative examples of ferronickel slag production, but the technical scope of the present invention is not limited to the examples. <Production example> Nickel oxide ore, recycled Ni raw material, carbonaceous material as a reducing agent, coal, and limestone as a slag melting agent were briquetted and charged into a rotary kiln under various conditions for reduction and smelting. The rotary kiln was 4.2 m in diameter, 72 m in length, and inclined at 1°, and operated at a rotation speed of 45 rph (revolutions per hour). The maximum temperature reached was 1300°C to 1500°C. The raw materials were heated using a pulverized coal burner from the front side of the kiln, i.e., the side from which the clinker was discharged. The raw coal used was semi-coke, with a consumption rate of 100 to 180 kg / ton of ore. The limestone was in the form of unburned calcium carbonate and was 10 to 100 kg / ton of ore. The resulting clinker, consisting of an agglomerate of ferronickel and slag, was water-cooled after being discharged from the rotary kiln. Thereafter, ferronickel particles were separated and recovered by crushing and underwater gravity separation, and ferronickel slag obtained as a residue was recovered.

[0035] The chromium in ferronickel slag comes from nickel ore and recycled Ni raw materials. While it is preferable to have less chromium, selecting only raw materials with low chromium content limits the types of raw materials that can be used. Therefore, raw materials were selected and blended so that the mass ratio of Cr2O3 in the entire burden was 5% or less.

[0036] The chemical composition of the ferronickel slag was analyzed using an X-ray fluorescence analyzer. The C and S concentrations were analyzed using the combustion infrared absorption method. The constituent mineral phases contained in the slag were identified using X-ray diffraction and a scanning electron microscope. Furthermore, the amount of hexavalent chromium eluted from the slag was measured based on the Environmental Agency Notification No. 46, which is the inspection method for soil environmental standards. The presence or absence of tumbling was confirmed visually inside the rotary kiln. The results are shown in Tables 1 to 3 as Nos. 1 to 17.

[0037] According to the experimental results shown in Table 1, in samples Nos. 1 to 17, which have suitable chemical composition ranges, mineral phases, and particles, the amount of hexavalent chromium eluted is within the standard value specified in Environment Agency Notification No. 46, and it can be seen that the chromium in the slag is fixed to the trivalent state.

[0038] <Comparative Example> The ferronickel slag of the comparative examples was also recovered as a by-product of ferronickel smelting, as in the production examples. It is shown as Nos. 18 to 29 in Tables 1 to 3. In the comparative examples, the chemical composition range or the contained mineral phase and contained particles were outside the ranges specified in the present invention, so the amount of hexavalent chromium elution exceeded the standard value in Notification No. 46 of the Environment Agency, or problems in ferronickel production were observed.

[0039] In No. 18, the SiO2 concentration in the ferronickel slag was too high, the slag basicity was too low, and the burned material in the rotary kiln did not roll sufficiently, causing the ferronickel particles to become fine. As a result, the ferronickel was not completely separated and remained in the slag, resulting in a high amount of hexavalent chromium elution. In addition, the insufficient rolling reduced the reduction efficiency and resulted in a high Ni content of 0.55 mass%, meaning that ferronickel recovery was insufficient and production costs increased.

[0040] The chemical composition of No. 19 was too low in SiO2 concentration and too high in MgO. As a result, the slag basicity was too high, and the (Mg,Fe)(Cr,Al)2O4 chromite phase was not formed. Furthermore, the slag melting point was high, and the burned material in the rotary kiln did not roll sufficiently, resulting in the ferronickel particles becoming finer. As a result, the ferronickel was not completely separated and remained in the slag. This resulted in a high amount of hexavalent chromium leaching. Furthermore, the insufficient rolling reduced the reduction efficiency, resulting in a high Ni concentration in the slag at 0.61% by mass. This meant that ferronickel recovery was insufficient, resulting in high production costs.

[0041] The chemical composition of No. 20 was too low in MgO and too high in CaO. As a result, the (Mg,Fe)(Cr,Al)2O4 chromite phase, which requires MgO, was not formed, and the amount of chromium contained in the CaO-SiO2-Al2O3-MgO-FeO amorphous phase increased. This resulted in a high amount of hexavalent chromium leaching. Furthermore, the increased liquid phase rate of the slag prevented sufficient rotation of the burned material in the rotary kiln. This resulted in a decrease in reduction efficiency and a high Ni concentration in the slag of 0.58% by mass. This meant insufficient recovery of ferronickel, resulting in high production costs.

[0042] The chemical composition of No. 21 had an Al2O3 concentration that was too low. As a result, a liquid phase was not formed in the slag, and the burned material in the rotary kiln did not roll sufficiently, causing the ferronickel particles to become finer. As a result, the ferronickel was not completely separated and remained in the slag. Furthermore, the (Mg,Fe)(Cr,Al)2O4 chromite phase was not formed. As a result, the amount of hexavalent chromium leached out was high. In addition, the insufficient rolling reduced the reduction efficiency, and the Ni concentration in the slag was high at 0.51% by mass. In other words, ferronickel recovery was insufficient, resulting in high production costs.

[0043] The chemical composition of No. 22 was too low in FeO concentration and too high in C concentration. As a result, the (Mg,Fe)(Cr,Al)2O4 chromite phase did not form, and the amount of chromium contained in the CaO-SiO2-Al2O3-MgO-FeO amorphous phase increased. This resulted in a high amount of hexavalent chromium leaching.

[0044] The chemical composition of No. 23 was too high in Cr2O3, which resulted in excess chromium entering the CaO-SiO2-Al2O3-MgO-FeO amorphous phase, exceeding the amount of chromium that could be fixed by the (Mg,Fe)(Cr,Al)2O4 chromite phase, resulting in a high amount of hexavalent chromium leaching.

[0045] The chemical composition of No. 24 had a low CaO concentration. As a result, no liquid phase was formed in the slag, and the burned material in the rotary kiln did not tumble sufficiently. This resulted in the ferronickel particles becoming finer, and the ferronickel was not completely separated and removed, remaining in the slag. This resulted in a high amount of hexavalent chromium leaching. Furthermore, the insufficient tumble reduced the reduction efficiency, and the Ni concentration in the slag was high at 0.55% by mass. This meant that ferronickel recovery was insufficient, resulting in high production costs.

[0046] The chemical composition of No. 25 had an excessively high Al2O3 concentration. This increased the liquid phase rate of the slag, and the burned material in the rotary kiln did not roll sufficiently, resulting in the ferronickel particles becoming finer. The ferronickel was not completely separated and remained in the slag. This resulted in a high amount of hexavalent chromium leaching. In addition, the insufficient rolling reduced the reduction efficiency, resulting in a high Ni concentration in the slag at 0.52% by mass. This meant that ferronickel recovery was insufficient, resulting in high production costs.

[0047] The chemical composition of No. 26 was too high in FeO and too low in C. This indicates that the rotary kiln had an oxidizing atmosphere, where chromium was easily oxidized to hexavalent chromium. This resulted in a high amount of hexavalent chromium leaching. Furthermore, the insufficient C concentration prevented sufficient reduction, resulting in a high Ni content in the slag (0.61% by mass). This meant that ferronickel recovery was insufficient, resulting in high production costs.

[0048] The chemical composition of No. 27 had an excessively high S concentration. As a result, the S concentration in the resulting ferronickel alloy was also high. This resulted in a high load on the refining process using AOD and VOD when producing stainless steel and special steel from ferronickel alloy, so it was deemed unsuitable.

[0049] The chemical composition of No. 28 was too low in Cr2O3, which meant that excessive carbon was used to reduce chromium to metallic chromium, resulting in high production costs.

[0050] The chemical composition of No. 29 was that the S concentration was too low. As a result, the ferronickel particles became finer, and the ferronickel could not be completely separated and removed, remaining in the slag. This resulted in a high amount of hexavalent chromium elution. In addition, because the ferronickel remained in the slag, the Ni concentration in the slag was high at 0.61 mass%. This meant that the recovery of ferronickel was insufficient, resulting in high production costs.

[0051] [Table 1]

[0052] [Table 2]

[0053] [Table 3] [Industrial Applicability]

[0054] Thus, the ferronickel slag of the present invention can fix most of the chromium oxide contained therein to trivalent chromium, and is suitable for use as a slag civil engineering material, and is therefore industrially useful.

Claims

1. The chemical composition on a dry mass basis is SiO ２ :30~70%, MgO:10~40%, CaO:1.0~10%, Al ２ O ３ :1.0~5.0%, FeO:1.0~20%, Cr ２ O ３ : 0.1 to 5%, S: 0.01 to 0.3%, C: 0.01 to 3%, Ni: 0.50% or less, and unavoidable components, SiO ２ The slag basicity, expressed as the mass ratio of MgO to (Mg, Fe) (Cr, Al) ２ O ４ Ferro-nickel slag consisting of slag particles containing chromite phase and unburned carbon particles.

2. The slag particles further comprise CaO—SiO ２ -Al ２ O ３ -MgO-FeO amorphous phase, (Mg,Fe)SiO ３ Pyroxene phase, and (Mg,Fe) ２ SiO ４ The ferronickel slag according to claim 1, which contains a silicate mineral phase consisting of three phases, including an olivine phase, as a main phase.

3. The slag comprises coarse-grained ferronickel slag and fine-grained ferronickel slag, The coarse-grained ferronickel slag has a mass fraction passing through a sieve with a nominal mesh size of 1.18 mm of 80 mass% or more and a mass fraction passing through a sieve with a nominal mesh size of 0.15 mm of 30 mass% or less, The ferronickel slag according to claim 1 or 2, wherein the fine ferronickel slag has a mass fraction passing through a sieve with a nominal mesh size of 0.075 mm of 60 mass% or more.

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