FURNACE FOR THE PRODUCTION OF FERROCHROMIUM ALLOYS

The object of this invention is a furnace, more specifically an arc furnace, for the production of ferrochromium alloys, of the type specified in the preamble to the first claim.

Ferrochromium alloys are currently in use.

These alloys are mainly composed of chromium, generally in the weight proportion over 40% (preferably between 50% and 70%), and the weight proportion of iron generally greater than 20% (preferably between 25% and 40%), as well as other elements in smaller percentages.

They are mainly used for the production of stainless steel.

Ferrochromium is generally produced from chromium ore, chromium oxide and iron, coal and/or coke.

The production process involves carbothermal reduction, which takes place at high temperatures, close to 2700°C, in which the chromium ore is reduced by coke to form the ferrochromium alloy.

The heat for this reaction is provided by an arc furnace. In these furnaces, heat is created by an electric arc formed between electrodes arranged in the furnace. This arc creates temperatures of about 2800°C.

The reaction causes a fusion of the minerals, creating a basin of ferrochromium in liquid state, above which a layer of slag is formed.

The material is poured from the furnace at intervals, through the casting holes on the side wall of the furnace.

When enough molten ferrochromium has accumulated in the furnace chamber, a casting hole is open and a stream of molten metal is poured into a vessel called a ladle.

The patent application PCT WO 201 1/013151 A1 describes a ferrochromium casting furnace with two casting holes, one for the ferrochromium and one for the slag formed above it. This solution aims to obtain better quality ferrochromium because less slag enters the alloy at the time of casting.

During operation, furnace walls are cooled by spraying water on the outside.

This well-known technique does include some significant drawbacks.

Indeed, ferrochromium alloys produced in this way often have significant impurities. For example, elements from the slag deteriorate in the alloy, such as silica.

Silica is added to the elements present in the furnace basin in the form of quartz, as it allows the acid slag to be maintained - a characteristic necessary for keeping the slag fluid and drainable.

In addition, the gases present in the molten alloy are frequently trapped therein and do not degas to the upper slag.

Another disadvantage is the water that cools the outside of the furnace. If this water is not collected correctly, it can form dangerous puddles which, in contact with the melted liquid, generate uncontrollable explosions.

In this context, the technical task behind this invention is to design a furnace for the production of ferrochromium alloys that is capable of substantially remedying at least part of the aforementioned problems.

One of the significant aims of the invention is to obtain a furnace for the production of ferrochromium alloys that enables the said alloys to be manufactured with low percentages of impurities.

Another important aim of the invention is to create a highly safe furnace for the production of ferrochromium alloys. The technical task and the specified aims are achieved by a furnace for the production of ferrochromium alloys as claimed in annexed Claim 1 .

The preferred technical solutions are highlighted in the associated claims.

The characteristics and advantages of the invention are clarified below, by the detailed description of preferred uses for the invention, with reference to the associated drawings, in which:

Figure 1 shows a middle section of the furnace for the production of ferrochromium alloys according to the invention;

Figure 2 is a horizontal section of the furnace for the production of ferrochromium alloys according to the invention;

Figure 3a shows a side view of a portion of the furnace for the production of ferrochromium alloys according to the invention; and

Figure 3b shows a top view of a portion of the furnace for the production of ferrochromium alloys according to the invention.

In this document, measurements, values, shapes and geometric references (such as perpendicularity and parallelism), when associated with words such as "about" or other similar terms, like "almost" or "substantially", are to be understood as being without measurement errors or inaccuracies due to production and/or manufacturing errors, and above all without slight divergences from the value, measurement, shape or geometric reference to which it is associated. For example, these terms, if associated with a value, ideally indicate a divergence of no more than 10% of the value.

Moreover, when used, terms such as "first", "second", "higher/upper", "lower", "primary" and "secondary" do not necessarily identify an order, a relationship priority or relative position, but can simply be used to more clearly distinguish between their different components.

The measurements and data contained in this text are to be considered, unless otherwise indicated, as carried out in ICAO International Standard Atmosphere (ISO 2533: 1975).

With reference to the Figures, a furnace for the production of ferrochromium alloys according to the invention is always indicated with the number 1.

Furnace 1 can be used for the production of ferrochromium alloys, in particular alloys composed mainly of chromium, in the weight proportion greater than 40% and preferably between 50% and 70%, and the weight proportion of iron greater than 20% and preferably between 25% and 40%.

With the exception of the elements described below, it is similar to the furnace and plant for ferrochromium described in patent application PCT WO 201 1/013151 A1 , in particular from page 5 line 19, to page 32 line 22 and in figures 1 , 2 and 3.

Furnace 1 for the production of ferrochromium alloys comprising: a base 10, and side walls 11 , preferably cylindrical.

They define an internal basin 12, comprising an inside base area 10a of the basin 12, on which the melted alloy will be placed.

Finally, the side walls define an external lateral surface 11a, preferably cylindrical. The inner basin should preferably be between 2.5 m and 3.5 m high and more preferably between 2.8 m and 3 m, the inner diameter should be between 7 m and 10 m and more preferably between 8 m and 9.5 m. Preferably, the diameter should increase from base area 10a to the upper surface.

Side walls 1 1 preferably have a thickness between 5 dm and 1 m and more preferably between 4 dm and 8 dm. They are preferably made of refractory bricks, which are in common use. Furnace 1 also includes electrodes 2 for the construction of an electric arc within inner basin 12.

The electrodes 2, of the commonly used type, are preferably made of graphite. There are preferably 3 of them and they are arranged vertically, substantially aligned and in the horizontal plane more or less according to the vertices of an equilateral triangle. This equilateral triangle, which touches the centre of the electrodes 2, has preferably a diameter between 3.5 m and 4.5 m.

The electrodes are also vertically movable, preferably up to a minimum height of 5.8 m to 6.5 m from base area 10a.

Furnace 1 also includes at least one casting hole 3 for casting the liquid ferrochromium alloy. There is preferably a first casting hole 3a, designed to pour the molten ferrochromium alloy, and a second casting hole 3b, placed vertically above the first casting hole 3a, designed for the slag.

The first casting hole 3a has the lowest portion, vertically, preferably at a distance of less than 5 cm from the base surface 10a, and more preferably substantially aligned with the base surface 10a.

In addition, the first casting hole 3a and the second casting hole 3b have an axis with reciprocal distance, in a vertical direction, preferably between 45 cm and 55 cm and more preferably between 48 cm and 52 cm.

The first casting hole 3a and the second casting hole 3b have a reciprocal angular distance, with their centre in the middle of the furnace, preferably between 90° and 120° and more preferably between 50° and 70°.

The casting holes 3 also have a normal section, preferably substantially circular. The diameter of the normal section of the 3 casting holes is preferably between 6 cm and 10 cm and more preferably close to 8 cm. The furnace 1 also preferably includes a heat sink 4. It consists of a structure made of metal material in direct contact with the outer lateral surface 1 1 a of furnace 1. The cooling ring is preferably made of a high-conductivity material, particularly copper alloy, or copper, or aluminium alloy, or aluminium.

It therefore has a ring shape that wraps around the outer walls 1 1 a, preferably with the exception of angular sectors in which the casting holes are present. The heat sink 4 is also suitably in direct contact with the refractory.

The normal section of the heat sink should preferably be between 1 m and 1 .7 m in height, and preferably between 1.2 m and 1 .5 m in height, and between 5 cm and 1 dm in thickness. It may include finned (Fig. 1 ) or grooved portions or irregularities to increase its external surface area, in order to maximise heat exchange.

Preferably, the heat sink 4 is composed of the tubes 40 in the materials indicated (Figures 3a and 3b) through which passes pressurised water, preferably substantially and homogeneously covering an axial portion of external walls 1 1 a. These tubes should preferably have an outer diameter of between 7 cm and 1 dm. The heat sink 4 is appropriately composed of modules 41 , each covering an angular portion of external wall 1 1 a and comprising an inlet 42 and an outlet 43 for water. There are also support elements 44 for attaching modules 41 to the wall of the furnace.

The heat sink 4 is preferably equipped with control valves for each section with thermal control probes to make the temperature of the corresponding sector more homogeneous. The internal water pressure is preferably between 2 atm and 4 atm. The heat sink 4 is preferably placed at least at the height, in the vertical direction, of the base surface 10a, or just below it. The heat sink 4 is preferably placed in the vertical direction at a lower height, at a distance between 3 dm and 5 dm, and more preferably between 35 cm and 42 cm, from the base surface 10a.

The operation of furnace 1 described above in structural terms is similar to the operation of furnaces that are in use and for example, described in patent application PCT WO 201 1/013151 A1 .

This operation also defines a new procedure for the production of ferrochromium alloys, i.e. by using furnace 1 described above, as well as a new use of furnace 1 described above for the production of ferrochromium alloys.

Furnace 1 according to the invention, as well as the new procedure and its new use, have significant advantages.

The heat sink 4 allows the furnace refractory to cool, thus also cooling the molten material and the slag, in a more appropriate way than the currently used water jets method. According to the applicant's experiments, the heat sink 4 homogenises the slag and alloy temperature more in the molten state, reducing the temperature difference between the two components.

The slag, positioned closer to the electrodes and their electric arc, usually has a higher temperature than the bath.

As the applicant discovered, this caused several problems: the melted alloy, at too low a temperature, did not degas correctly and kept gaseous impurities inside it, while the slag, at too high a temperature, melted the quartz (present to keep the slag acidic and fluid) and deteriorated the silica in the melted alloy, further worsening its quality.

By reducing the thermal delta between the two portions, the described heat sink 4 enables the temperature of the molten alloy to be increased, allowing for better degassing and decreasing slag temperature to below 1750°C, avoiding the melting of the quartz and its deterioration, in the form of silica, in the molten alloy. This advantage has been further increased by the reduced vertical distance between the first and second casting holes 3a and 3b. This reduced distance, brought to the described values, has enabled the height of the electrodes to be further lowered from the base surface 10a, keeping them closer to the molten metal and further reducing the thermal delta.

For example, under normal conditions, the Chromium efficiency in standard processes is between 75 and 85%, meaning that the Chromium efficiency is the ratio between the one placed in the furnace in the form of a mineral and the one contained in the metal obtained in the form of a mineral. Using furnace 1 described above led to a chromium furnace yield of more than 92%. This value is very close to the stoichiometric value imposed during the calculation of the charge composition. Furnace 1 therefore also allows for a significant reduction in electricity consumption, which depends on the Cr203 mineral content, slag quantity, the degree of refractoriness of the chromite and the correct conduction of the furnace.

Finally, the invention has achieved another significant benefit - that of greater security for the operating staff. Since the use of water jets is not strictly necessary, there is no longer the risk of explosions due to the instantaneous vaporisation of water that accidentally comes into contact with the molten liquid.

The invention is susceptible to variants within the aim of the inventive concept defined by the claims. In this context, all details may be substituted by equivalent elements and the materials, shapes, and sizes may vary greatly.