Metallurgical Process and Plant herefor

This invention relates to a metallurgical process for the production and/or treatment of at least one metal or ore in a reactor from which a waste gas stream loaded with molten particles and/or vaporous constituents is discharged via a waste gas conduit and subjected to an aftertreatment by supplying at least one cooling gas or cooling gas mixture. This invention furthermore relates to a plant for performing this process.

In metallurgical processes, waste gases from electric furnaces or similar reactors frequently contain molten particles and/or vaporous constituents, which upon cooling and/or oxidation tend to form very fine solid particles which can clog the waste gas conduit by formation of crusts. Such problems resulting from the clogging of the waste gas conduit are known for instance from slag cleaning furnaces in the copper industry, electric furnaces in the secondary metallurgic industry (copper, lead), or plasma furnaces in the production of titanium dioxide slag (TiO ₂ slag).

To prevent this formation of crusts, it has been proposed to add air to the waste gas stream in the waste gas conduit in a controlled or uncontrolled manner. Such process is known for instance from EP 0 661 507 A1 , wherein a cooled partial stream of a clean gas is admixed to the waste gas stream before its entry into a gas cleaning plant. According to EP 0 661 507 A1 , dust particles can additionally be introduced into the waste gas, which serve as condensation nuclei. By condensation of volatile metal compounds on the dust particles, while at the same time cooling the waste gas below a temperature which can lead to the particles being sintered on the inner wall surfaces of the gas conduits, it should be avoided that condensates are deposited on these inner wall surfaces.

In practice, however, this process has not led to satisfactory results, so that the deposits in the waste gas conduits still have to be removed "by mining" in regular intervals.

Therefore, it is the object of the present invention to propose an improved process and a plant herefor, which largely avoids the formation of crusts on the inner wall surface of a waste gas conduit of a metallurgical reactor by molten particles and/or vaporous constituents entrained in the waste gas stream.

In accordance with the invention, this object substantially is solved with a process as mentioned above in that on the side of the waste gas conduit facing the reactor a cooling gas or cooling gas mixture, the temperature of which is lower than that of the waste gas stream, is injected into the waste gas conduit with a high velocity and substantially tangentially. The waste gas conduit thus acts as a gas-gas cooler, by which the hot waste gas can be cooled to a temperature below the condensation and sublimation temperature of the vaporous constituents of the waste gas stream and below the melting temperature of the molten particles. By tangentially injecting the cooling gas or cooling gas mixture, the hot waste gas stream is kept away from the inner wall surface of the waste gas conduit by forming a stable layer of cooling gas between the inner wall surface and the waste gas stream, whereby the formation of crusts can be avoided. Tangentially supplying a cooling gas or cooling gas mixture into the waste gas conduit has the additional advantage that the material of the waste gas conduit is exposed to a considerably lower thermal load. This provides for using inex- pensive materials for the waste gas conduit, so that the process of the invention not only is particularly favorable due to lower maintenance costs in operation, but also provides for a less expensive manufacture of the plant components.

In accordance with a preferred embodiment of the invention, the cooling gas or cooling gas mixture injected into the waste gas conduit is air waste gas. The use

of ambient air is recommendable, as the same is available at low cost and with a comparatively low temperature. If no ambient air should be used for cooling the waste gas stream, for instance to avoid reactions of the waste gas stream with the atmospheric oxygen, it is preferred, however, in some applications to use cleaned and in particular cooled waste gas for cooling the waste gas stream in the waste gas conduit. If the waste gas to be used as the cooling gas has a lower temperature than the waste gas from the metallurgical reactor, it can also be used without cooling and washing/cleaning.

In a particular embodiment of the invention, a cooling gas or cooling gas mixtures is used for cooling, the properties of which, e.g. composition, thermal capacity or viscosity, are adapted corresponding to the requirements. For instance, water-saturated, cleaned and cooled waste gas can be used for cooling. In addition, e.g. the water vapour content of the gas can be adjusted, for in- stance by specifically adding water droplets. The cleaned waste gas can also be mixed with air or gases or cooled waste gases from other parts of the plant.

In accordance with a development of the invention, it is preferred to inject the cooling gas or cooling gas mixture into the waste gas conduit such that at least in a region of the waste gas conduit facing the reactor, a layer of the cooling gas or cooling gas mixture is formed on the inner wall surface of said conduit. Thus, the cooling gas or cooling gas mixture cooling the waste gas stream keeps the waste gas stream not only away from the inner wall surface of the waste gas conduit, but forms a layer which prevents a direct contact between the waste gas stream and the inner wall surface. This leads to a particularly efficient reduction of the formation of crusts in the waste gas conduit, since tacky and doughy particles do not reach the inner wall surface. Also, the temperature of the wall itself is very low and close to the temperature of the cooling gas, so that dust particles adhering to the wall are not molten or get sticky by contact with the wall.

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In accordance with the invention, the cooling gas or cooling gas mixture injected into the waste gas conduit has a temperature of less than about 200 ⁰ C, preferably below about 80 ⁰ C. In practice, it was found to be useful when the tempera- ture of the cooling gas or cooling gas mixture injected into the waste gas conduit lies between about 0 ⁰ C and about 160 ⁰ C, preferably between 25°C and 150°C. Preferably, the temperature of the cooling gas or cooling gas mixture injected into the waste gas conduit is chosen in dependence on the temperature of the waste gas stream. In accordance with a preferred embodiment of the invention, the temperature of the gas or gas mixture can lie at least about 100 K, preferably at least 500 K below the temperature of the waste gas stream at the reactor outlet.

To sufficiently keep the waste gas stream away from the inner wall surface of the waste gas conduit and to form a layer enveloping the waste gas stream between the inner wall surface and the waste gas stream, the gas or gas mixture is injected into the waste gas conduit with a high velocity. The gas velocity of the gas or gas mixture should lie at least 50% above the gas velocity of the waste gas stream in the waste gas conduit. Preferably, the gas velocity of the gas or gas mixture injected into the waste gas conduit is between about 20 and 100 m/s, in particular between 30 and 90 m/s, and preferably between 40 and 60 m/s.

Another feature of the gas-gas-cooler is the mixing of and heat exchange be- tween the hot waste gas on the one hand and the cooling gas on the other hand. Ideally the two streams are completely mixed and have assumed a homogeneous mixing temperature at the end of the gas-gas-cooler.

When the waste gas conduit acting as gas-gas cooler is formed only compara- tively short and/or the cooling performance is not sufficient, clogging of the

waste gas conduit itself can largely be prevented, but there is a risk that parts of the plant downstream of the waste gas conduit or the feed conduits are clogged by adhering molten particles or sticky dust formed out of vaporous constituents by condensation/sublimation. In the process of the invention, the quantity and temperature of the cooling gas or cooling gas mixture injected into the waste gas conduit therefore preferably is adjusted to the temperature and quantity of the waste gas stream and/or to the material properties of the molten particles or vaporous constituents of the waste gas stream such that the highest temperature of the mixed gas stream leaving the gas-gas cooler lies below the melting temperature of the molten particles and/or below the condensation temperature of the vaporous constituents of the waste gas stream. It may be necessary to cool below a critical temperature at which the dusts begin to become sticky. This temperature often is below the melting point of the molten particles. In practical applications, the two gas streams are not completely mixed and the length of the waste gas conduit acting as gas-gas cooler is chosen such that the temperature difference between the hot waste gas stream and the enveloping layer at the end of the waste gas conduit facing away from the reactor lies below 100 K, preferably below 60 K.

In accordance with this invention, the term vaporous constituents preferably refers to those vaporous constituents of the waste gas which upon cooling can condense or sublime in the waste gas conduit, i.e. whose condensation/sublimation point or condensation/sublimation temperature lies below the reactor temperature, but above the gas temperature in the waste gas conduit, preferably the gas temperature of the cooling gas supplied.

The process of the invention is suitable for cooling the waste gas of different metallurgical processes, in which the waste gas is loaded with molten particles and/or vaporous constituents. The process of the invention advantageously can be used in particular for slag cleaning furnaces in the copper and nickel indus-

tries, in electric furnaces in the secondary metallurgy (copper, lead), and for plasma/electric furnaces in the production of TiO ₂ slag. Further fields of use include e.g. the electric furnaces and the converters for producing steel, stainless steel, pig iron and ferro-alloys, electric-resistance furnaces, electric-arc furnaces, blast furnaces or melt reduction furnaces. For instance, ferro-alloys such as ferronickel, ferrochromium can be produced in an electric-resistance furnace or electric-arc furnace.

The object underlying the invention furthermore is solved by a plant which is suitable for performing the process described above. In accordance with the invention, this plant includes a reactor for the treatment of ores and/or metals and a waste gas aftertreatment stage associated to the reactor for cooling and/or cleaning a waste gas stream from the reactor, which is loaded with molten particles and/or vaporous constituents. To form a gas-gas cooler, the waste gas aftertreatment stage is equipped with a supply means for supplying cooling gas or a cooling gas mixture into a waste gas conduit for the waste gas stream. In accordance with the invention, the supply means is formed and arranged such that at least in a portion of the waste gas conduit facing the reactor the cooling gas or cooling gas mixture forms a gas layer enveloping the waste gas stream on the inner wall surface of the waste gas conduit. This layer should be present at least as long as the temperature of the waste gas is below the critical temperature mentioned above. This configuration of the supply means of the plant in accordance with the invention effects that the waste gas conduit cannot be clogged with the molten particles or vaporous constituents of the waste gas, that the cooling gas layer enveloping the waste gas stream forms a layer between the waste gas stream and the inner wall surface of the waste gas conduit, which largely prevents a contact of the waste gas stream with the inner wall surface and cools the inner wall surface.

In accordance with the invention, this can be achieved particularly efficiently in that the supply means includes a nozzle assembly and/or a confusor for injecting cooling gas or a cooling gas mixture into the waste gas conduit with a high velocity, wherein the nozzle assembly is aligned relative to the waste gas con- duit such that the cooling gas or cooling gas mixture can be injected into the waste gas conduit substantially tangentially with respect to the main direction of the waste gas stream. The nozzle assembly can for instance be arranged annu- larly around the waste gas conduit with a plurality of individual nozzles. Alternatively, it is also possible to provide lances or the like, which extend into the waste gas conduit and are aligned such that a substantially tangential flow of the cooling gas or cooling gas mixture into the waste gas conduit can be effected. The confusor preferably has a slotted opening, whose dimension in longitudinal direction of the waste gas conduit is greater than in radial direction.

To be able to efficiently avoid crusts or deposits in the waste gas conduit, it is furthermore preferred that the waste gas conduit is formed substantially rectilinear at least in a first portion downstream of the supply means. Hence it is possible that a particularly stable layer enveloping the waste gas stream can be formed out of the tangentially injected cooling gas or cooling gas mixture. Only when the waste gas stream has been cooled to such an extent that the risk of clogging of the waste gas conduit is minimized, can the conduit change direction, e.g. by means of a segmented elbow.

For this purpose, the waste gas conduit acting as gas-gas cooler has a sufficient length, which in many applications is at least 4 to 8 times the diameter. It is preferred, however, that the straight portion of the waste gas conduit has a length of at least 5 to 6 times the diameter. The supply means also can be formed such that the cooling gas or cooling gas mixture is not only introduced into the waste gas conduit at one point, but is distributed along a certain length of the waste gas conduit by means of a confusor. Alternatively or in addition, it is

also possible to provide a plurality of supply means spaced from each other along the straight portion of the waste gas conduit, in order to maintain a stable layer enveloping the waste gas stream.

A particularly inexpensive design of the plant in accordance with the invention becomes possible if at least the waste gas conduit is made of carbon steel or a low alloy steel. Without the invention, the waste gas conduit would have to be made of a particularly temperature-resistant and therefore expensive material, in order to be able to safely discharge the hot waste gas stream, which frequently has temperatures above 1000 ⁰ C. In accordance with the invention the gas layer enveloping the waste gas stream provides for a defined cooling of the inner wall surface of the waste gas conduit, which allows to use less expensive, not high- temperature-resistant materials for the waste gas conduit.

In accordance with one embodiment of the invention, a blower for supplying cooling gas, e.g. air, and/or for supplying cooled and/or cleaned waste gas is associated to the supply means. Hence it is achieved that via the supply means the cooling gas or cooling gas mixture can be introduced into the waste gas conduit with a high velocity, whereby a gas layer enveloping the waste gas stream can be formed along a particularly long distance. Upon cleaning in a gas washer downstream of the waste gas conduit, part of the waste gas stream can be recycled, potentially after further cooling, in order to again be introduced into the waste gas conduit via the supply means as a cooling gas mixture.

Further features, advantages and possible applications of the present invention can be taken from the following description of an embodiment with reference to the drawing. All features described and/or illustrated per se form the subject- matter of the invention, also independent of their inclusion in the claims or their back-references.

In the drawing:

Fig. 1 schematically shows a plant for performing a metallurgical process in accordance with the invention,

Fig. 2 schematically shows a side view of a confusor for a plant in accordance with the invention, and

Fig. 3 schematically shows a section through the confusor of Fig. 2.

On the left side in Figure 1 , a suspension furnace 1 is indicated, in which, e.g. for obtaining copper, concentrate is molten by adding silicates to obtain copper matte and an iron silicate slag. The slag, which contains e.g. about 2% copper, is supplied to a slag cleaning furnace 3, as indicated by the arrow 2 in Figure 1. The copper matte, which has a high copper content, then is withdrawn from the suspension furnace 1 , as indicated by the arrow 4.

From the slag cleaning furnace 3, copper matte is withdrawn, as indicated by the arrow 5. The slag designated by the arrow 6, which is discharged from the slag cleaning furnace 3, only has a low copper content of e.g. less than 1 %.

The waste gases of the slag cleaning furnace 3 are discharged through a waste gas conduit 7. The waste gases here can have a temperature of e.g. about 1200 ⁰ C. The waste gas stream flowing through the waste gas conduit 7 in part is loaded with molten particles and vaporous constituents. Downstream of the waste gas conduit 7, there is provided a waste gas treatment stage indicated by the arrow 8, which for instance includes a non-illustrated gas washer and/or further cooling stages.

To avoid clogging of the waste gas conduit 7 by the molten particles or vaporous constituents entrained in the hot waste gas stream, a cooling gas stream indicated by the arrow 9 is injected with a high velocity on the side of the waste gas conduit 7 facing the slag cleaning furnace 3. This is effected for instance via a confusor 10 shown in Figures 2 and 3 or via a nozzle assembly such that the cooling gas is introduced into the waste gas conduit 7 substantially tangentially, so that a veil or a cooling gas layer enveloping the waste gas stream is formed on the inner wall surface of the waste gas conduit 7.

Along the waste gas conduit 7, which for instance can have a length of about 17 m, the cooling gas stream enveloping the waste gas stream gradually mixes with the waste gas, so that on the side facing away from the slag cleaning furnace 3 the waste gas stream is cooled for instance from about 1200 ⁰ C to about 500 ⁰ C to 600°C. The waste gas conduit 7 thus forms a gas-gas cooler for the waste gases from the slag cleaning furnace 3.

The quantity and temperature of the cooling gas stream 9 injected into the waste gas conduit 7 is adjusted to the temperature and the quantity of the waste gas stream and the material properties of the molten particles or vaporous constitu- ents entrained therein such that the mixing temperature of the waste gas stream and of the cooling gas stream 9 lies below the melting temperature of the molten particles or below the condensation temperature of the vaporous constituents of the waste gas stream, whereby a formation of crusts in the waste gas conduit 7 can be avoided.

The length of the gas-gas cooler results from the required distance which is necessary, until a certain mixing combined with temperature compensation of the two gas streams has occurred inside the waste gas conduit 7. A deflection of the waste gas stream only is effected after this temperature compensation is

effected, i.e. at the end of the waste gas conduit 7 facing away from the slag cleaning furnace 3.

Figure 2 shows a preferred embodiment of the introduction of the cooling gas stream into the waste gas conduit by means of a confusor 10. By way of example, there is only shown a supply conduit for cooling gas at one point. The cooling gas conduit 9 tangentially enters into the waste gas conduit 7 at the edge thereof. In this special embodiment, the waste gas conduit 7 has a reduced cross-section at the entry point.

Figure 3 shows a section through the confusor 10 of Figure 2, wherein in the illustrated embodiment the cooling gas conduit 9 is tapered before entry into the waste gas conduit. The cooling gas then is tangentially introduced into the waste gas conduit 7.

List of Reference Numerals:

1 suspension furnace 2 slag

3 slag cleaning furnace

4 matte

5 matte

6 slag 7 waste gas conduit (gas-gas cooler)

8 waste gas aftertreatment

9 cooling gas supply

10 confusor