GB2043695A - Rotary kiln treatment of metal- bearing slurries - Google Patents

Abstract

In a process for obtaining metals from steel plant wastes, an aqueous slurry of the waste is charged directly into the upper end of an inclined rotatable kiln 15 in which it is fired together with a carbonaceous reductant to produce a highly metallic sponge iron residue from which lead and zinc have been volatilised, and discharging the sponge iron from the lower end. <IMAGE>

Description

SPECIFICATION Obtaining metals from steelplant wastes This invention relates to a process for obtaining metals from steelplant wastes arising in dust or slurried form. More particularly the invention is directed to the production of pre-reduced iron (sponge iron) from such wastes and the recovery of lead and zinc.

From one aspect the present invention relates to a process for obtaining metals from steel plant wastes, wherein an aqueous slurry of said waste is charged directly into the upper end of an inclined rotatable kiln in which it is fired together with a carbonaceous reductant whereby to produce a highly metallic sponge iron residue from which lead and zinc have been volatilised, and discharging the sponge iron from the lower end.

The slurry may primarily be ferruginous waste products such as in-plant fines from ironm akingl steelmaking processes or rolling mill waste, e.g.

millscale, but coke or coal is preferably added to aid the reduction stage. Limestone may be added to adjust the basicityofthefeed material as necessary.

Make-up water may also be added to obtain the right consistency.

The reductant may be solid, e.g. coke and/or coal, introduced into the kiln part way along its length and the combustion air may be injected at the lower end.

The gaseous products of combustion carried up through the kiln, which include the lead and zinc, issue through the upper end into a drop-out chamber or cyclone in which the heavier combustion products are removed. A significant proportion of this is 'Waelz oxide' - rich on lead and zinc - but a higher grade of this material is obtained from an electrostatic precipitator or bag filter through which the gases then pass.

The discharged sponge iron which is in the form of agglomerates (granules) or powder depending on the process parameters is cooled and screened into three or more product streams.

Many processes of manufacturing sponge iron have been utilised hitherto with a rotary kiln but these have involved charging dried and blended "green" balled dusts or pelletised metalliferous material (which may have been derived from a slurry). With the pelletising process there are limitations on the size of the particles in the blend. With this invention however a slurry, which may be the naturally occurring waste, is charged direct so that no pre-processing is necessary leading to a considerably more economic process.

In order that the invention may be fully understood one embodiment thereof will now be described by way of example, with reference to the accompanying drawing which schematically illustrates the process route in accordance with this invention.

Referring now to the drawing a series of hoppers contain primary and secondary feed materials. The primary feed materials comprise the metalliferous steelplant waste in a series of hoppers (or bins) 1 and coke breeze in a hopper 2. The hoppers 1 contain for example arc furnace dusts from various steelworks either in sludge or powder form, basic oxygen system waste scurries and other in-plant fines as desired. The secondary feed materials comprise re-cycled low grade 'Waelz oxide' 3 and sponge iron rejects - when present - 4. Bentonite 5 and/or limestone 6 may be added as required.

Various quantities of these materials are continuously fed by a dumping mechanism (schematically illustrated at 7) into a wash mill 8 comprising a tank 9 in which a number of gates 10 are suspended and slowly rotated for stirring the contents. A quantity of 'make-up' water 12 is discharged into the tank to maintain the correct consistency. Specifically, the specific gravity may be about 1.8, the water content being about 42%.

The slurryfrom the wash mill is discharged into storage tanks 13 from whence it is fed either direct to a 'spoon' feeder 14 - for discharge into a rotary kiln 15 - or via a further slurry storage tank 16 having paddles 17 for agitation. An overflow feedback loop 18 is provided from the spoon feeder back into the tank 16.

The rotary kiln 15 is of the type commonly used for cement-making purposes comprising an inclined hollow refractory clad shell 15A rotatable at about /2 r.p.m. by a drive mechanism 15B. The total length of this particular shell is about 60 metres with a central necked portion of about 3 metres diameter between two larger diameter 'belled' areas, one at the upper end for receiving the slurry and the other at the lower end downstream of the carbonaceous reductant charging port. A curtain of chains is mounted in the upper section of the kiln. The reductantfuel is charged through an annular port 19 and comprises a mixture of coke breeze, coal and recycled char fed from hoppers 20,21, and 22, respectively, along a conveyor 23.A certain amount of air is allowed to enter the kiln through these ports but the main combustion air is injected through a nozzle 24 sited in a fixed housing 25 and projecting towards the lower end of the kiln. The charge material migrates through the kiln and as it passes through the curtain of chains it is dried and nodulised, it is then exposed to the reaction zone at about 1200or and issues through the lower end as iron units in the reduced or beneficiated state, surplus fuel (char) and ash. This material is then spray cooled by water as it passes through an inclined cooler 26 and separated by a mesh screen 27 which recovers the sponge iron larger than 6 mm at 27A the steam and gaseous matter from this cooler is vented through a venturi scrubber 28.

The smaller fraction of the sponge iron, i.e. - 6mm, is recovered by a magnetic separator 29 at 29A the remainder of the material being fed through a mesh screen 30 from which char is recovered for recycling through hopper 22, the smaller size ash can be filtered through this screen at 31 for dumping.

The combustion products issuing through the upper end of the kiln include volatilized zinc and lead identified as 'Waelz oxide', so called here because of its similarity to the recovery of these elements in the well known Waelz process, together with various alkalis. The heavier components in this suspended matter drop out in a chamber 32 (these contain in total about 40%-50% lead and zinc) whilst the remaining combustion products are recovered in an electrostatic precipitator 33 at 34. A much higher grade of material is recovered here (about 55%-60% lead and zinc) and this is pelletised or otherwise agglomerated for further use whereas the lower grade of this material from chamber 32 is recycled through the system via hopper 3.

The equipment described is capable of processing 8 tonnes per hour of ferruginous wastes with an estimated output of 5.2 tonnes per hour of sponge iron, containing over 60% metallic iron, and 1.4 tonnes per hour of high grade Waelz oxide.

Although this invention has been described with reference to the particular embodiment illustrated it is to be understood that various modifications may readily be made without departing from the scope of the invention. For example, the reductant has been shown to be introduced gravitationally part way along the length of the kiln but alternatively the carbonaceous matter may be injected up the kiln by compressed air. Further, the combustion air may be closely controlled in orderto minimise the entrainment of fines in the kiln which leads to a diminution in the yield of sponge iron and dilution of the Waelz oxide by ferrous matter. In order to aid the drop-out of the latter, the chamber 32 may embody a water spray in the roof.

It is not essential for the kiln to be of the type commonly used for cement making, i.e. as shown and with chain curtains, etc; other, more conventional reduction furnaces may alternatively be used.

Claims (11)

1. A process for obtaining metals from steelplant wastes, wherein an aqueous slurry of said waste is charged directly into the upper end of an inclined rotatable kiln in which it is fired together with a carbonaceous reductant whereby to produce a highly metallic sponge iron residue from which lead and zinc has been volatilised, and discharging the sponge iron from the lower end.

2. A process according to claim 1, in which the volatilised products issue through the upper end of the kiln and are filtered to recover the lead and zinc as Waelz oxide.

3. A process according to claim 2, in which the recovery is effected in two stages, the coarser material being recovered in a drop-out chamber or a cyclone disposed in advance of an electrostatic precipitator or bag filter in which the finer fractions are recovered.

4. A process according to any one of claims 1 to 3, in which, following discharge, the sponge iron is extracted by a magnetic separator and recovered in different sized fractions by a mesh screen.

5. A process according to any one of claims 1 to 4, in which some of the residues from the materials recovered are re-cycled and discharged into the aqueous slurry.

6. A process according to any one of claims 1 to 5, in which the slurry primarily consists of ferruginous waste arising from ironmaking/steelmaking processes together with millscale and coke and/or coal, and make-up water.

7. A process according to any one of claims 1 to 6, in which the reductant is charged into the kiln part-way along its length, the main combustion air being injected through a nozzle projecting upwardly towards the lower end of the kiln.

8. A process according to claim 7, in which the reductant is coal and/or coke.

9. A process according to any one of claims 1 to 8, in which the material discharged from the lower end of the kiln is spray-cooled in an inclined cooler from which the gaseous products are vented.

10. A process for obtaining metalsfromsteelplant wastes, substantially as herein described - with reference to the accompanying drawings.

11. Apparatus for performing a processfor obtaining metals from steelplant wastes, substantially as herein described - with reference to the accompanying drawing.