Smelting processes, for instance smelting processes in the steel and metal manufacturing industries, emit contaminated process gases from which solid contaminants are normally extracted in a filter system prior to releasing the gas to atmosphere. The solids extracted may be a mixture of so- called mechanical dust (for instance droplets of iron, slag, powdered limestone, etc.) and so-called chemical dust formed by the condensation of vaporised metals to form metallic fumes. The metallic fumes, or smoke, may contain iron often together with minor quantities of zinc, lead and arsenic, for instance.

For instance, dust that derives from steel smelting processes in which scrap metal is involved will normally contain 0.5-8% zinc. This percentage of contaminating zinc is much too high for the dust to be recycled back to the steel process while, other the other hand, being much too low for the dust to be of any economic interest with respect to zinc production. The dust thus generated is normally dumped at present. At the present time, smaller quantities of dust are processed in novel expensive (purification) processes. It is generally known, however, that dumping dust that derives from metal smelters creates economic and ecological problems. The dumping costs vary greatly from one country to another, and discussions are being held as to whether or not to introduce environment fees that will bring dumping costs on a level

with the costs of processing or treating the solid contaminants.

One object of the present invention is to provide a highly attractive method for purifying dust-laden gases. Another object is to provide an arrangement for carrying out the inventive method. These objects are achieved with a method and an arrangement having the features set forth in the following Claims.

The invention affords many advantages, of which some are given below.

If desired, extracted solids can be made available for recirculation and processing in the original process or in a parallel process.

The invention provides a method of selectively extracting metals that have mutually different vaporisation temperatures, such as zinc, cadmium, arsenic, antimony and bismuth, for instance, from hot contaminated process gases downstream of metal smelters, rubbish incinerators, etc.

Several different metals can be separated selectively from one another, by means of a serial arrangement of inventive apparatus.

The inventive arrangement can be readily integrated in a process gas flow.

The invention has both technical and economical advantages.

The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which Figure 1 is a vertical sectional view of an inventive arrangement; Figure 2 illustrates schematically a metal smelter and shows the process gas flow supplemented with an inventive arrangement and peripheral equipment; Figure 3 is a schematic illustration of two inventive arrangements operating in series; Figures 4 and 5 are schematic illustrations of alternative embodiments with respect to the peripheral equipment of the inventive arrangement; and Figure 6 shows an example of a flowchart relating to a metal smelting process in which the inventive method is applied.

Figure 1 illustrates an inventive arrangement 1 in the form of a so-called metal condenser. The arrangement 1 includes a vertical channel or column 2 that has a process-gas inlet 3 at its bottom end and a process-gas outlet 4 at its upper end. The condenser 1 also includes at its upper end an inlet 5 for vapour condensing elements 10 and at its bottom end an outlet 6 for said condensing elements 10. The condensing elements 10 exiting from the outlet 6 are collected in a collecting vessel 7 beneath said outlet.

The inventive arrangement 1 operates and is used in the following way.

Assume that the arrangement 1 is coupled in a process-gas flow 100, for instance deriving from a so-called LD-furnace for steel manufacture.

Contaminated and hot process gases depart from the LD- furnace. The total amount of solids or dust present in the process gas may, for instance, be about 20 kg per tonne of steel produced. About 20% of steel scrap is normally charged to the LD-furnace as a coolant for cooling and regulating the additional energy developed in the process. In addition to containing iron, solid contaminants deriving from the LD- process and its scrap addition will normally contain between 0.5 and 8% zinc. This percentage of zinc is much too high to enable the dust to be cycled back to the steel process while being much too low for the dust to be of any economic interest with regard to zinc manufacture.

Assuming that the hot process-gas flow 100 contains zinc and that it is desired to extract the zinc, which is in a gaseous state down to a temperature of about 750°C, by means of the inventive arrangement 1.

This is achieved in accordance with the inventive method by allowing the hot process gas 100 and its gaseous zinc content to come into direct contact with relatively cold surfaces of condenser elements introduced into the process-gas flow. The zinc therewith condenses on the colder surfaces whereas, e. g., iron particles that have traversed to a solid state at temperatures above 1000°C are not affected, but accompany the process gas to downstream particle separators.

In the aforedescribed exemplified case, zinc is thus extracted selectively from the hot process gas and forms a zinc concentrate that can be processed beneficially in known zinc processing apparatus, whereas the iron particles that

are no longer contaminated by the zinc can be returned as raw material to the steel manufacturing process.

According to the inventive method illustrated in Figure 1, hot process gas 100 having a temperature such that the zinc present will be in a gaseous state while the iron is in a solid state (particle state) flows through the space 2 while a plurality of relatively cold condenser elements 10 are introduced through the inlet 5. In the illustrated case, the elements 10 fall through the space 2 gravitationally and exit through the outlet 6. As the elements fall down through the space 2 and through the gas 100 flowing through said space, the gaseous zinc present in the gas 100 will condense on the cold surfaces of the elements 10, so that the elements 10 will be coated with zinc when leaving the space 2 through the outlet 6. These zinc coatings have been designated 11 in Figure 1. As the elements 10 exit through the outlet 6, they are collected, e. g., in a collecting vessel 7 and, e. g., cooled down and cycled back to the inlet 5. This recycling of the elements 10 can be repeated an appropriate number of times. When suitable, the zinc-coated elements 10 can be transferred to a separate zinc processing apparatus and therewith be replaced by other elements 10 that have been cooled to the intended temperature and that are supplied to the arrangement 1 through the inlet 5, and so on. It should be mentioned that with respect to recovery, the elements 10 will preferably be comprised of the same material as that which is to be condensed from the gas, i. e. the elements 10 will preferably be made of zinc in the illustrated case. It will be understood, however, that the size, shape, number and material content and temperature of the elements 10 may be varied as required and desired. The process gas 101 that has

been substantially cleansed of its zinc content leaves the arrangement through the outlet 4, for further processing.

Figure 2 illustrates by way of example how peripheral equipment associated with an inventive arrangement 1 can be disposed. The process gas 100 departing from a steel furnace 20, e. g. an LD-furnace, passes through an inventive arrangement 1 where a gaseous metal is extracted from the process gas by condensing on elements 10 in the aforedescribed manner. The elements 10 are stored in a container 21 and dispensed/metered into the space 2 in the condenser arrangement 1, via the inlet 5. The elements 10 fall through the space 2 and become coated with condensate and leave the space 2 via the outlet 6, whereafter said elements are cooled to a desired temperature in a cooling bath 22 and forwarded to a screen or sieve 24 on a conveyor 23, where the condensate-covered elements 10 are sorted with respect to size and those elements 10 that are to be recycled in the condensing process by means of a conveyor/elevator 25 are transported back to the container 21 for renewed introduction into the space 2 through which the process gas flows, and so on. Cleansed process gas 101 leaves the arrangement 1 for further processing.

Figure 3 illustrates an embodiment of the invention where two inventive arrangements 1 and 1'are mutually connected in series, wherewith a first metal (e. g. zinc) can be extracted from the process gas 100 in the first arrangement 1 by means of first elements 10, as seen in the direction of gas flow, while a second metal that has a lower condensation temperature than said first metal can be extracted from the process gas 101 leaving the first arrangement 1 by virtue of

its condensation on second elements 10'. The process gas 102 cleansed in the second arrangement 1'leaves the second arrangement 1'through the outlet 4'for further processing as desired. Naturally, the temperature of the process gas 100-102 and the temperature of the elements 10 and 10'in question will be adapted to achieve desired extraction of selected metals by condensation. It will also be understood that more than two inventive arrangements 1 can be connected in series when the need arises. Alternatively, inventive arrangements may be connected in parallel.

Figure 4 illustrates an alternative embodiment of the invention with regard to movement of the elements 10 through the process-gas flow. In this case, the condenser elements 10 are moved with the aid of a gas-permeable conveyor 30 that runs through the gas-flow space 2 in the arrangement 1.

Cooling, selection and/or recycling, etc., of the elements 10 takes place outside the space 2.

Figure 5 illustrates a further embodiment of the invention with respect to movement of the condenser elements 10 through the process-gas flow. In this case, the condenser elements are moved through the flow of process gas in a pendulating fashion in two gas-permeable element packs 10a and lOb, wherein one pack of elements, 10a, is located within the space 2 and is through-passed by said gas whilst the other pack, 10b, is located outside said space 2, where the elements are cooled and the condensate 11 removed therefrom, and vice versa.

Figure 6 is a flowchart that exemplifies a steel manufacturing process in which an inventive method 50 is

applied, wherewith zinc concentrate leaves the process for appropriate recovery or some other process.

It will be understood that the invention of course can be applied in conjunction with cleansing gas flows that derive from many different types of processes and that the invention is not restricted to use in conjunction with any specific process.

It will also be understood that the design of the inventive arrangement can be varied widely within the scope of the inventive concept. Non-limiting examples of such variations are component replacements, variations in the direction of process gas flow, etc.

The peripheral equipment of the inventive arrangement can also be designed in many ways different to those exemplified above.

It will also be understood that many variations are possible with respect to the elements 10, and the elements 10 can be caused to pass through the gas flow solely once prior to leaving the inventive arrangement for further processing if so desired, even though recycling of said elements is normally to be preferred. The elements 10 may also be comprised of a material different to that to be extracted from the process gas by means of the inventive condensation method.

The invention is thus not restricted to the illustrated and described embodiments thereof, since changes and modifications can be made within the scope of the following Claims.