WO2001040528A1 - Post-treatment method for deoxidizing metallurgical slag - Google Patents

Abstract

The invention concerns post-treatment method for metallurgical slag containing metal oxides which are difficult to deoxidize. The molten slag is subjected to a treatment to reduce the metal oxides contained therein and to elutriate the deoxidized metal products. The deoxidizing treatment is carried out on a highly carburized metal bath starter, thereby promoting the reduction of metal oxides which are hard to deoxidize with carbon. Advantageously the carburized metal bath starter is at a temperature not less than 1450 DEG C, obtainable by overheating the slag with electric arc(s) on said carburized metal bath starter.

Description

 DAIRY REDUCING AFTER-TREATMENT PROCESS

METTALLURGIQUES

Introduction

The present invention relates to a process for post-treatment of metallurgical slag containing metal oxides which are difficult to reduce

State of the art

It has long been known to use blast furnace slag as a raw material, for example in road construction, in the manufacture of cement or in the production of fertilizers. However, as regards the recovery of slag from electric steelworks, satisfactory solutions are not yet available. One reason is in particular the presence of large quantities of impurities in these slags, in particular metal oxides. The slag from steelworks producing stainless steels is particularly problematic. Indeed, these slag contains large quantities of chromium oxides, which we know are both leachable and toxic. It follows that the slag from steelworks stainless steel cannot be recovered directly and furthermore constitute a pollution hazard when they are placed in dirt. In order to be able to recover slag from electric steelworks, it has been proposed to subject these slag to a metallurgical post-treatment, in which the metal oxides contained in the slag are subjected to a reduction reaction in the molten slag Once the reduction reaction is complete, the metal fraction is separated from the mineral fraction by melt decantation The purified mineral fraction in fusion is then granulated to be used as an alternative to blast furnace slag The metal fraction can also be used praised

In known methods of metallurgical post-treatment of slag, the molten slag is poured into a metallurgical furnace (for example an arc furnace electric), possibly together with other production waste rich in metal oxides (dust, sludge,) The slag is then heated by simultaneously adding carbon to it, to reduce in a first step the easily reducible metal oxides, in particular the oxides of iron For slags rich in metal oxides that are difficult to reduce, such as slag from stainless steel mills rich in oxides of chromium, manganese, tungsten, vanadium and molybdenum, it has been proposed to submit the slag, after reduction of the iron oxides by carbon, in a second reduction step During this second step, more powerful reducers than carbon are added to the slag, while continuing to heat it Or, powerful reducers which are suitable for this second reduction step , such as for example silicon in the form of ferro-silicon (FeSi) or calcium in the form of carbide (CaC ₂ ), are much more expensive than carbon, which makes the profitability of the process doubtful, especially if in the presence of large quantities of oxides which are difficult to reduce in the slag, the consumption of these reducing agents becomes significant

Subject of the invention

The object of the present invention is to improve the efficiency of a process for post-treatment of metallurgical slag containing hardly reducible metal oxides, in which the molten slag is subjected to a reducing treatment to reduce the oxides metallic, and the metallic products are decanted from said reducing treatment

Summary of the invention.

In accordance with the invention, this objective is achieved by a process in which the reducing treatment is carried out on a carburetted metal bath foot, generally containing at least 2.5% of carbon and preferably more than 4% of carbon II will be appreciated that this carburetted metal bath foot constitutes a reducing medium and a reserve of thermal energy which promote a reduction of metal oxides, such as chromium oxides, which are considered to be difficult to reduce by carbon. This notably makes it possible to reduce the consumption of reducing agents which are more expensive than carbon. The carburetion of the metal bath foot is preferably carried out by injecting carbon fines into the metal using a submerged lance. This avoids significant foaming of the slag, which would hinder the progress of the reduction and decantation stages.

The mass of the carburetted metal bath foot is at least equal to the mass of the slag to be treated on this bath foot. Its temperature is at least 1450 ° C during the reducing treatment. The metal bath foot therefore constitutes an important source of heat, which makes it possible to guarantee the progress of the reduction operation at high temperature, thereby promoting a reduction by carbon of the metal oxides considered to be difficult to reduce. It is preferably performed overheating by electric arc (s) of the slag on the carburetted metal bath foot, whose large mass also has a stabilizing effect on the arcs. To increase the efficiency of this heating without disturbing the course of the process, the slag can be foamed locally in the zone of the arc or arcs, for example by targeted injection of oxygen and carbon into the zone or arcs.

It is advantageous to stop the overheating by electric arc (s), respectively to significantly reduce its power, when a temperature above 1500 ° C is reached in the slag. Next, a mixing of the metal / slag interface is started, so as to have a reduction in the metal oxides of the superheated slag in contact with the carburetted metal bath foot. The mixing of the metal / slag interface can for example comprise a bubbling of neutral gas.

Decantation of reduced metal products is advantageously activated by gentle mixing of the metal / slag interface. It will be appreciated that dust or sludge rich in metal oxides to the slag to be treated, which is for example a slag rich in Cr ₂ O ₃

A process according to the invention is normally carried out as follows a) loading of the liquid slag to be treated in an arc furnace in which there is a large metal bath foot, b) overheating of the slag and the metal bath foot by arc (s ) electric ^), c) during said overheating, carburetion of the metal bath foot by injection of a carbonaceous product, and d) after said overheating, mixing of the metal / slag interface, so as to have a reduction of oxides metals from the superheated slag in contact with the metallic fuel bath, e) decantation of the reduced metal products, and f) pouring of the purified slag

Example

Other features and characteristics of the invention will emerge from the detailed description of an example of application of the method, which is provided below by way of illustration.

For the implementation of the method, an electric submerged arc furnace is preferably used, generally called a submerged arc furnace (SAF) furnace. It is a fixed bowl furnace of fairly simple design, which is poured at through a tap hole, which is closed using a capper and opened using a drilling machine. However, a free arc furnace could also be used, as it is for example used for melting scrap.

The product to be treated is a mixture of stainless steel slag and dust, roughly having the following composition 6% CR ₂ O ₃ , 8% (FeO +

MnO), 84% (CaO + SιO ₂ + AI ₂ O ₃ ) It has a CaO / SιO2 basicity of approximately 1.5 (possibly adjusted by adding lime) Remain to note that in the continuation one will also use the term “slag” to indicate the mixture “slag-dust” to treat

A batch of approximately 15 tonnes of this slag, which has a temperature of approximately 1200 ° C., is loaded into the arc furnace on a metal bath foot having a mass at least equal to the mass of the slag to be treated (the mass of the metal bath foot will gradually increase with the number of treatments which are carried out on the metal bath foot) The metal bath foot is at a temperature of approximately 1450 ° C. Its composition is approximately as follows 2.5-3% of carbon, 0-30% chromium (the chromium content of the metal bath foot will gradually increase with the number of treatments that are carried out on the metal bath foot), the rest being mainly iron Remains that the bath foot metallic is normally already covered with an initial layer of slag with a height of about 20 cm Indeed, during a slag pouring, a layer of slag is allowed to float on the metal bath foot, in order to avoid to flow from the m stall with the slag On this residual layer of slag we pour the batch of slag to be treated

In the next step, the slag to be treated and the metal bath foot on which it floats are heated by electric arc The heating power is approximately 10 to 12 MW, the electrical energy consumed is 600 to 800 kWh The appreciable mass of the metal bath foot guarantees excellent arc stability

It should be noted that in an SAF oven, the thermal energy is generated by conduction of the current in the liquid slag. There is consequently no plasma arc (or free arc) proper, which is established in the electrodes. and the metal bath In order to increase the efficiency of the heating operation, the layer of slag is foamed locally around the electrode, so as to create around it a local layer of foaming slag in which the density slag is at least 50% weaker than in the rest of the oven If the oven has several electrodes, it is preferable to foam the layer of slag locally around all the electrodes of the furnace It will be appreciated that the creation of a local layer of foaming slag changes the shape of the passage of electrical energy in the bath We pass, at least partially, from a current conduction electric in liquid resistive slag, to a “plasma” arc formed in a gaseous medium, even if this medium also includes a certain proportion of liquid slag In this way one can increase the characteristics of arc, ie arc voltage and arc length The electric field in plasma mode immersed in foaming slag is at least two to four times greater than in resistive mode (conduction in liquid slag) This results in a significant increase in the power of the SAF oven On the other hand, it will be noted that the majority of the slag remains in a liquid state (that is to say non-foaming), which will have a favorable influence on the heat conduction da ns the slag and the kinetics of the reduction and decantation reactions described below Remain to note that the local layer of foaming slag surrounding the electrodes is advantageously formed by targeted injection of oxygen and carbon in the zone of the arc (s) The reaction carbon with oxygen produces CO which froths the slag

Simultaneously with the heating, a carburation of the metal bath is carried out in order to raise the carbon content of the metal bath from 2.5% to approximately 6.5%. This carburation is advantageously carried out using one or more lances immersed in the molten metal to inject carbon in the form of fines or granules pneumatically in the molten metal It will be appreciated that the injection of carbon into the metal bath using immersed lances avoids producing an excessively foaming slag, which would have an unfavorable influence on the subsequent process

In the context of the example described HERE, 600 to 800 kg of carbon are injected in the form of fines with a flow rate of approximately 20 kg / min, which corresponds to a duration of approximately 30 to 40 mm for the operation. During this time, the electric arc heating raises the temperature of the slag from 1200 ° C to a temperature of about 1550 to 1600 ° C. It remains to be noted that at the start of the heating / carburation stage, it may be interesting to introduce carbon also in the slag, on condition however of not producing a slag which is still foaming at the end of this stage

As soon as a slag temperature above 1550 ° C and a carbon concentration of about 6.5% in the metal bath has been reached, the local foaming operation is interrupted, the electric arcs are cut off and the heating is stopped. carburation operation of the metal bath The next step is a reduction treatment in which the reducible oxides of the superheated slag react with the carbon in solution in the metal bath This reaction consequently requires an intense slag-metal exchange, which is activated by stirring of the metal-slag interface This stirring is carried out in a known manner by bubbling neutral gas (nitrogen or argon) injected into the metal, either by porous brick in the hearth, or by submerged lance, or by a combination of two It will be appreciated that this reaction between the highly carburized liquid metal and the superheated liquid slag does not only make it possible to reduce the easily reducible metal oxides ibles, such as iron and manganese oxides, but also hardly reducible metal oxides, in particular chromium oxides The flow rate of stirring gas during this reduction step is approximately 100 to 200 l / min for a period about 15 min. It will be noted that at the end of the reduction step, a small amount of a powerful reducing agent can optionally be added to the slag. In the example described, for example, 15 kg of FeSi is added to the slag.

After this intense stirring step of the metal-slag interface, there follows a step of 10 to 15 mm of softer stirring, in which the stirring gas flow is around 50 l / min. This gentle stirring promotes the settling of the metallic products obtained by reduction of the metallic oxides of the slag, which pass through the metallic bath base

The purified slag is then poured at a temperature of 1500 ° C. through the taphole of the oven. As mentioned above, a layer of slag of about 20 cm is left during this pouring on the metal bath foot, this in order to avoid pouring metal with the slag After 8 to 10 slag flows, we will also have to pour a metal, to free up space in the oven

The purified slag contains less than 0.5% of Cr ₂ O ₃ and less than 0.3% of FeO, that is to say that the process made it possible to extract from the slag more than 90% of the chromium and more of 97% of the iron We consumed approximately 700 kg of C and 15 kg of FeSi (added at the end of reduction), as well as approximately 5500 kWh of electricity With a conventional process we would have consumed approximately 500 kg of C and 500 kg of FeSi to obtain the same result Since FeSi is approximately five times more expensive than carbon in fine, it will be understood that the proposed process has an important economic advantage. It remains to be noted that in the process described above, the stirring of the metal-slag interface after reaching the desired final temperature and carbon content (i.e. around 1550 ° C and around 6.5% of C) It may however be interesting to start already stirring of the interface metal-slag before having reached these desired final values (for example after having reached a temperature of about 1450 ° C in the slag and a carbon content of 3 to 4% of C in the metal bath foot) In this case it can also be advantageous to lower the heating power when reaches for example 1500 ° C and just keep a holding power

Claims

claims 1. A method of post-treatment of metallurgical slag containing metal oxides which are difficult to reduce, in which the molten slag is subjected to a reducing treatment to reduce the metal oxides, and the metal products are decanted from said reducing treatment, characterized in that the 'the reducing treatment is carried out on a carburetted metal bath foot. 2. Method according to claim 1, characterized in that the carburetted metal bath foot contains more than 2.5% of carbon. 3. Method according to claim 2, characterized in that the carburetted metal bath foot contains more than 4% of carbon. 4. Method according to any one of claims 1 to 3, characterized by carburizing the metal bath foot by injecting carbon fines into the metal using a submerged lance. 5. Method according to any one of claims 1 to 4, characterized in that the mass of the carburetted metal bath foot is at least equal to the mass of the slag to be treated on this bath foot. 6. Method according to any one of claims 1 to 5, characterized in that the carburetted metal bath foot is at a temperature of at least 1450 ° C during the reducing treatment. 7. Method according to any one of claims 1 to 6, characterized by an overheating by arc (s) electric (s) of the slag on said metal carbide bath foot. 8. Method according to claim 7, characterized by localized foaming of the slag in the area of the arc (s). 9. Method according to claim 8, characterized in that said localized foaming is obtained by targeted injection of oxygen and carbon in the region of the arc (s). Method according to any one of Claims 7 to 9, characterized by an injection of carbon into the slag limited to the start of said overheating by electric arc (s) Method according to any one of Claims 7 to 10, characterized in that said overheating by arc (s) electrιque (s) is stopped, respectively its power is significantly reduced, when reaching a temperature above 1500 ° C, and then begins a mixing of the metal / slag interface, so as to have a reduction in the metal oxides of the superheated slag in contact with the carburetted metal bath foot Method according to claim 11, characterized in that said mixing of the metal / slag interface comprises a bubbling of neutral gas Method according to one any of claims 1 to 12, characterized by decantation of the reduced metal products activated by gentle mixing of the metal / slag interface Method according to any one elconque of claims 1 to 13, characterized in that dust or sludge rich in metal oxides is added to the slag to be treated Method according to any one of claims 1 to 14, characterized in that the slag to be treated is a slag rich in chromium oxide Method according to any one of Claims 1 to 15, characterized by the following stages a) loading of the liquid slag to be treated in an arc furnace in which there is a metal bath foot, b) overheating of the slag and of the metal bath foot by electric arc (s) ^), c) during said overheating, carburetion of the metal bath foot by jection of a carbon product; and d) after said overheating, mixing of the metal / slag interface, so as to have a reduction in the metal oxides of the slag superheated in contact with the carburetted metal bath; e) settling of reduced metal products; and f) pouring the purified slag.