It is an object of the present invention to provide a method for recovering chromium in slag which can reduce chromium contained in slag, improve powderization due to expansion, and use slag as a resource. This method of recovering chromium in slag is performed using a smelting furnace.
Slag generated during decarburization refining of stainless steel Litter into a ladle containing hot metal that has been subjected to pretreatment such as dephosphorization And raises the heat to reduce the chromium oxide in the slag. This method increases the liquid phase ratio of the slag, Chromium oxide by reduction of chromium oxide Wear. And productivity is reduced. SiO 2 can be suppressed.
When the basicity is lower than 2. When the basicity is higher than 7. The reduction reaction can be promoted by positively contacting the slag with the metal Al contained in the hot metal or aluminum dross. As shown in FIG. Furnace lid 13 Is lifted and lowered entirely by a lifting device not shown , and furthermore, the electrode 14 can be raised and lowered independently in the ladle The unreduced slag 15 whose O has been adjusted to 2.
Aluminum dross, dolomite, Mg such as MgO brick debris By adding O-containing material, Al 2 in the slag 15 at the end of reduction is added. The addition amount of the alumidroth or the MgO-containing material is determined such that the amount of slag discharged into the ladle 11 is grasped, and the total concentration of Al 2 O 3 and MgO becomes a predetermined concentration.
Since the liquid phase ratio of the slag 15 is increased, the mass transfer of chromium in the slag 15 is promoted, and chromium oxide Cr 2 O 3 in the slag 15 is rapidly reduced by the metal Al in the added aluminum dross.
In addition, the generated chromium Cr can be efficiently recovered in the hot metal In addition, since the slag 15 has a basicity within a predetermined range and is added with Al 2 O 3 , a calcium aluminate-based compound can be generated, and free CaO Can be suppressed and expansion can be reduced. The hot metal 16 from which chromium has been recovered is used as a source of iron for the next charge in decarburization refining in a refining furnace such as an upper-bottom-blow converter, an upper-blow converter or an electric furnace.
The slag 15 is cooled after being transported to the treatment plant, crushed, and then used as a landfill material for civil engineering, a roadbed material, and other aggregates.
Examples of the method will be described. Before desulfurization, dephosphorization, etc. Soshi The slag generated during the decarburization refining beforehand for dephosphorization etc.
Table 1 shows the results. Example Example 1 and Example 2 were to make all the above conditions within the scope of the present invention. Is reached, Two O Three 0. In Example 5, the lower limit of the basicity is 2. Cr Two O Three Concentration can be 0. During decarburization, additions are made for obtaining the proper final chemical composition. These additions usually consist of desired amounts of high carbon ferrochromium, stainless steel scrap, carbon steel scrap, nickel, iron, high carbon ferromanganese, and molybdenum oxide.
These additions also serve to reduce the bath temperature as carbon and chromium oxidations are exothermic. In general, the bath temperature is controlled to less than deg C. During the final stage of blowing, the ratio of oxygen to argon is changed to to for bringing carbon to the desired value which can be less than 0. The next step is the reduction step, in which the reduction additions are charged and stirred with an inert gas for a desired time.
The bath is then stirred with inert gas, typically for around five to eight minutes. Additional silicon addition is needed if requirement of silicon is there to meet the silicon specification of some of the stainless steels.
Careful manipulation of slag, as it precipitates in the reaction, is important. Any chromium oxide not reduced by carbon ends up in the slag, which can form a complex spinel.
The effectiveness of reduction step is dependent on many factors including slag basicity and composition, temperature, mixing conditions in the converter and solid addition dissolution kinetics.
During the oxygen blow, silicon is oxidized before carbon. Lime and dolomitic lime are sometimes added before the end of the blow to cool the bath and to reduce the volume of reduction additions. The formation of a high basicity slag and the reduction of oxygen potential in the metal bath are good conditions for sulfur removal.
For example, with starting sulphur of 0. The length of the blow period is determined by the starting carbon and silicon levels of the hot metal charged to the AOD converter. Decarburization time ranges from 20 to 35 minutes in modern converters start from 1. Usually, the converter is turned down to a horizontal position and a sample of the liquid steel is taken for analyses at a carbon level of about 0.
Sulfur removal is a slag — metal reaction that occurs during the reduction phase of the process. Phosphorus, which requires oxidizing conditions, cannot be removed in the converter processing. Nitrogen control is a gas — metal reaction. Depending on final nitrogen specification for the stainless steel grade, the inert gas during the initial stages of decarburization can be nitrogen. After a certain carbon level is achieved, the nitrogen gas is replaced by argon. Such an approach is usually practiced by steelmakers to reduce argon usage and costs and still achieve a desired nitrogen specification.
After the change from nitrogen to argon, nitrogen is removed from the bath both by evolved carbon monoxide and argon. Volatile elements with high vapour pressures, such as lead, zinc, and bismuth, are removed during the decarburization period.
The formation of high basic slag and the reduction of oxygen potential in the liquid steel bath are good conditions for sulphur removal. These are achieved by having a high lime concentration in the slag and a low oxygen activity in the metal bath. The transfer of sulphur to slag takes place as per the following reaction.
Additions of lime are made to dilute the sulphur in the liquid steel bath. Also, aluminum or silicon may be added to remove oxygen. For example, with a start sulphur of 0. If the grade to be produced requires an extra low sulphur level, the bath is deslagged after the reduction step and another basic slag is added. The liquid steel and the fluxes are then mixed to complete the desulfurization reaction.
In modern practices a sulphur level of 0. Other trimming alloy additions might be added at the end of the step. After sulphur levels have been achieved the slag is removed from the AOD vessel and the metal bath is ready for tapping. Ideally at this stage of the process, the chemistry of the liquid steel should meet the final specifications so that the heat can be tapped.
If necessary, additional raw materials may be charged for small chemistry adjustments before tapping. After tapping, the ladle is often stirred for composition homogenization and temperature uniformity along with flotation of inclusions.
This is done in a ladle equipped with stirring facilities with or without the use of a ladle furnace. After the ladle treatment, the steel is ready to be cast. In the early days of the AOD process, the converter was tilted for raw material additions as well as for taking samples and for measurement of temperature using immersion thermocouples. The vessel itself is designed for optimized flat-bath operation, which guarantees highest carbon removal efficiency CRE.
Therefore, the bath geometry is a compromise between low bath depth for reduced ferrostatic pressure and sufficient bath depth for ensured mixing and chemical reaction of injected process gases. A robust tilting drive with two asynchronous motors ensures fast and precise tilting of the AOD converter. Feedback from several reference installations proves that this damping system reduces dynamic load and converter displacement, which entails a range of benefits: reduced fatigue and wear on equipment especially on gear wheels and bearings , increased equipment lifetime, less maintenance effort, and increased operational safety.
In order to facilitate fast vessel exchange, the setup utilizes the Vaicon Quick suspension. First, an exchange device is placed on the empty ladle-transfer car. This device then lifts out the worn AOD vessel and transports it into the crane bay, where it is transferred into the maintenance area.
This newly established part of the plant mainly consists of a converter-wrecking stand where the old lining is removed, and a combined converter relining and heating stand where the new lining is installed and the AOD vessel is heated up before going into production again.
Charging of the main additions is handled by a newly installed additive-feeding system above the AOD converter, while scrap is charged into the AOD by means of a scrap chute. The new alloy and additive feeding system is operating out of three storage areas, which combine pre-existing with new storage facilities. In order to achieve the lowest-possible emission levels, advanced primary and secondary dedusting systems featuring pulse-jet filter technology were installed for the new AOD process aggregates.
Moreover, to improve the dust situation inside the steel plant, the new dedusting system is also equipped with several suction points for the existing EAF material handling system, the ladle furnace, the treatment stand, and the torch-cutting machine.
The cooling system necessary for primary AOD off-gas cleaning builds on new, patent-pending technology, which is more effective, requires fewer fans and less space, minimizes dilution air and heat flow to the filter, and thus significantly reduces operating and maintenance costs. A crucial factor for accurate and efficient process execution is the process-optimization and control software, also referred to as Level 2 and Level 1 automation.
The patented dynamic process model employed at the SIJ Acroni plant comprises a number of metallurgical sub-models, which are combined to perform online calculations and offline simulations of the state of the bath and the chemical reactions occurring inside the vessel. The installation had been performed while regular production at the facility was ongoing—shutdown times were kept to a minimum. In the weeks and months following the first tap, almost 20 different stainless and special grades were successfully produced with the new setup.
Overcoming the bottleneck. Steelmaking Acroni. Authors Authors Bernhard Voraberger. The main advantages of the AOD converter in this regard are: Potential for using less expensive charging materials without compromising quality. Sign up for our Newsletter.
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