GB2196881A

GB2196881A - Cleaning and cooling metallurgical exit gases

Info

Publication number: GB2196881A
Application number: GB8725500A
Authority: GB
Inventors: John Gabb Philip
Original assignee: IMP SMELTING PROCESSES
Current assignee: IMP SMELTING PROCESSES
Priority date: 1986-10-31
Filing date: 1987-10-30
Publication date: 1988-05-11
Anticipated expiration: 2007-10-30
Also published as: GB8626087D0; GB2196881B; GB8725500D0; IN169342B

Abstract

Exit gases from pyro-metallurgical processes are cooled and cleaned and condensible vapours in the gases condensed by flowing them tangentially through a hot-gas cyclone the interior walls of which are irrigated with a molten metal, alloy or salt which serves to remove gas-borne liquid and solid particles by impingement on the wetted walls of the cyclone.

Description

SPECIFICATION Cleaning and cooling of metallurgical exit gases This invention relates to the cleaning and cooling of exit gases emanating from pyrometallurgical processes e.g. blast furnaces, flash-smelters or fuming furnaces for producing metal products.

In many pyro-metallurgical processes exit gases leaving the process may be contaminated with dust particles, liquid droplets, fume and condensible vapours.

For example in the blast-furnace smelting of zinc, exit gases from the "lead-splash" condenser, which is used to condense zinc vapour from the off-gases, contains lead droplets, zinc fume, zinc fog and solid oxidic particles (dust).

Similarly in the lead industry, exit gases can contain lead fume, droplets and dust, the latter being variously metallic, oxidic, sulphidic or sulphated in form. Examples are found in suspension smelting, flame cyclone smelting, electric furnace smelting and slag-bath smelting.

Treatment of such exit gas generally involves optional heat recovery followed by gas cleaning to recover valuable metal values and prevent pollution of the environment.

Amongst the main problems encountered in handling such exit gases are the following; (a) build-up of material in the cleaning/cooling apparatus (b) blockage of ducts and passages as the gas temperature is reduced (c) blockage at the wet/dry line in wetscrubbing equipment.

It is an object of the present invention to treat such exit gases in such a way that a large proportion of dust, droplets, fume and condensible vapours are removed from the gases, while minimizing the problems mentioned above; so that a cooled and cleaned gas is available for further processing e.g. filtration, heat recovery and burning to produce sensible heat.

Known apparatus for such use usually includes large cross-section and height gas-offtake stacks which are used in order to reduce gas velocities and to allow settlement of gasentrained liquids and solids. In certain cases, such as that for the QSL direct lead smelting process, the stack walls are indirectly cooled to reduce the temperature of gases to a level below which solids are no longer sticky and do not foul the subsequent heat recovery operations. However, the stack walls build-up with material that must be removed by mechanical means.

In one aspect this invention consists in apparatus for cleaning, cooling or condensing exit gases or vapours from pyro-metallurgical processes, which apparatus comprises at least one hot-gas cyclone having a tangential gasinlet, wherein the interior walls of the cyclone are positively irrigated with a molten-phase material which serves to remove gas-borne liquid and solid particles by impingement on the wetted walls of the cyclone.

Preferably the irrigating molten phase is caused to flow, by gravity, from top to bottom of the interior walls of the cyclone.

Preferably the gas flows countercurrent to the flow of the molten phase, which practically means gas entering (tangentially) at the bottom of the cyclone and the molten phase running down the internal walls of the cyclone, from top to bottom.

Preferably the irrigating molten phase is sprayed through the gas stream within the cyclone on to the walls.

The molten phase used for irrigation preferably has the following properties; low volatility; reasonably low melting point; high temperature capability; low corrosivity; compatibility with the gas stream components.

In a second aspect the invention consists in a method of cleaning, cooling or condensing gases and/or vapours leaving a pyrometallurgical process comprising feeding said gases tangentially to a hot gas cyclone and contacting them with a film of a molten phase fed to the internal walls of the cyclone.

The molten phase is arranged to irrigate the walls of the cyclone by one of several means.

In a preferred embodiment the irrigating liquid is injected tangentially at the top of the cyclone to swirl down the walls in co- or contra-direction to that of the gas stream,thus insuring complete coverage with no dry spots that might form accretion. In another method the irrigating liquid might be introduced as an axial falling film, running down the walls. In yet another method the irrigating liquid might be sprayed onto the walls to give complete irrigation. The molten phase is optionally sprayed into the gas stream such that enhanced cooling and cleaning takes place.

In the case of the zinc blast furnace condenser exit gases the irrigating liquid is preferably splash-condenser lead extracted at a suitable temperature and zinc activity to be pumped into the gas cleaning cyclone. In this case there is no cooling of the gas because the lead stream temperature will be at, or above, the gas stream temperature. The major requirement is the separation of the fine lead droplets and dross from the condenser offgas.

The irrigation lead is returned to the lead splash-condenser circuit where captured solids are removed with existing equipment.

In a further application to the zinc blast furnace or other zinc vapour generating process, the irrigated cyclone has the attributes of good gas-vapour contact for removal of con densible vapours and can be used to perform all or part of the zinc vapour condensation duty in addition to the removal of particulate material. Thus, one or more, of the stages of condensation of the zinc blast furnace condenser can be replaced with one or more linked irrigated cyclones to absorb zinc, scrub particulates and cool the gases. Similarly, other zinc vapour generators such as plasma or flame reactors that have a high dust content in the zinc-containing off-gas can utilize the rapid inertial separation of solids at the same time as absorbing zinc into the irrigating liquid.

This will minimize the opportunity for reversion of zinc vapour to zinc oxide to occur on solid dust surfaces which is a well known source of inefficiency in zinc condensation practice.

In the case of other metallurgical processes the irrigating liquid is preferably a metal or metallic alloy in a separate closed circuit such that removal of captured material and heat is independent of the main process.

For lead smelting processes the irrigating phase is preferably essentially lead so that captured material with some irrigating liquid entrainment can be returned to the main process without contamination problems or loss of valuable alloying component.

For tin-treating metallurgical processes the irrigating phase might be lead-tin alloys to enable lower melting points to be utilized yet return any entrained alloy in captured material to a suitable point in the flowsheet. The use of alloys may also be applicable in general situations where entrainment is minor.

For copper-treating metallurgical processes the irrigating phase must be chosen to suit the requirements. For example, in the Mitsubishi Continuous Copper Process the smelting stage has a difficulty with solid build-up in the furnace offtake and the waste heat boiler inlet.

This results in downtime to remove the buildup by oxygen lances or explosives. The buildup is mainly slag entrained in the gas stream by the turbulent action of the smelting bath, but other phases could be present. Thus if lead or lead alloys are chosen as the irrigating phase, in a cyclone apparatus according to the invention the solubilization of captured phases should be minimized by maintaining a reasonably low temperature.

The inlet temperature of the gas stream to the invention may cover a wide range depending on the temperature capability and circulation rate of the irrigating phase. Thus for lowmelting alloys the inlet gas temperature could be as low as 2000C and for low volatility liquids, the inlet gas temperature could be as high as 1 2000C or more.

Heat extraction rate from the device will vary with the mean temperature difference of the gas and molten phase irrigating liquid. Removal of heat from the exit irrigating liquid in the case of metals or alloys can be accomplished according to present art by water cooled panels immersed in the said irrigating liquid to generate hot water, process steam or working fluid for prime movers or electricity generation. Thus the energy of the gas stream can be partially recovered in useful form by this invention in the same manner as heat is extracted from the irrigating liquid lead stream of the zinc blast furnace condenser.

The materials of construction of the working surfaces of the cyclone will depend on the temperature and corrosivity of the system. For example at moderate temperatures it may prove sufficient to use steel or special alloy working surfaces whereas at high temperatures refractory working surfaces may be required. The maximum working temperature of the irrigating phase is envisaged in the 600 to 700 C range and is therefore not particularly onerous. The working surfaces must preferably be smooth to ensure uniform coverage by the irrigating liquid.

In addition to the irrigating liquids mentioned above it may prove possible in some circumstances to consider the use of molten salts for the irrigating duty.

Claims

1. Apparatus for cleaning, cooling or condensing exit gases or vapours leaving pyrometallurgical processes, comprising at least one hot-gas cyclone having a tangential gasinlet, and means for positively irrigating the interior walls of the cyclone with a molten phase material.

2. Apparatus as claimed in claim 1 wherein the gas is caused to flow countercurrent to the molten phase which is caused to run down the internal walls of the cyclone under gravity.

3. Apparatus as claimed in claims 1 or 2 wherein the molten phase is fed tangentially to the top of the cyclone and the gas is fed tangentially to the bottom and flows countercurrent to the flow of the molten phase.

4. Apparatus as claimed in claims 1 or 2 wherein the molten phase runs down the internal walls of the cyclone as an axial falling film.

5. Apparatus as claimed in claims 1 or 2 wherein the molten phase is sprayed on to the internal walls of the cyclone to irrigate the said internal walls.

6. A method of cleaning, cooling or condensing gases and/or vapours leaving a pyrometallurgical process comprising feeding said gases tangentially to a hot gas cyclone and contacting them with a film of a molten phase fed to the internal walls of the cyclone.

7. A method as claimed in claim 6 wherein the gas stream flows countercurrent to a falling film of liquid on the internal wall of the cyclone.

8. A method as claimed in claim 6 wherein the molten phase is fed tangentially to the upper inner wall of the cyclone and the gas is fed to the bottom of the cyclone.

9. A method as claimed in claim 6 wherein the molten phase is fed as an axial falling film to the internal walls of the cyclone.

10. A method as claimed in claim 6 wherein the molten phase is sprayed from the gasspace within the cyclone on to the cyclone walls.

11. A method as claimed in claims 6 to 10 wherein the molten phase is a molten metal or alloy.

12. A method of cooling, cleaning or condensing gases or vapours from a pyrometallurgical process substantially as hereinbefore described.

13. Apparatus for cleaning, cooling or condensing vapours leaving a pyro-metallurgical process substantially as hereinbefore described.