GB2043112A - Recovery of metals from wastes - Google Patents

Abstract

A process for obtaining metals from metalliferous wastes particularly steel-plate wastes in which an aqueous slurry of the waste (10) is subjected to high pressure filtration (11) and the resultant cake charged into a kiln (14) where it is reduced with a carbonaceous material. There is discharged from the lower end of the kiln a highly metallic sponge iron residue (17) from which an oxide rich in lead and zinc has been volatilized and recovered from the upper end, the cake possessing a sufficiently high moisture content as to enable the upper end kiln temperature to be at a level low enough to permit the gaseous products to be filtered direct (23, 24) without the provision of specific cooling means. <IMAGE>

Description

SPECIFICATION Recovering metals from metalliferous wastes This invention relates to a process for obtaining metals from metalliferous wastes arising in dust or slurried form, e.g. steelplant wastes.

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, but it is to be understood that any metallic material may be produced dependent on the nature of the feedstock and other volatiles recovered.

From one aspect the present invention relates to a process for obtaining metals from metalliferous wastes, wherein an aqueous slurry of said waste is subjected to high pressure filtration, in which the resultant compacted waste is charged into the upper end of an inclined or upright kiln where it is fired together with a carbonaceous reductant, whereby there is discharged from the lower end a highly metallic sponge iron residue from which an oxide rich in lead and zinc has been volatilised and recovered from the upper end, the cake possessing a sufficiently high moisture content as to enable the upper end kiln temperature to be at a level low enough to permit gaseous products from the kiln to be filtered direct without the provision of any significant specific cooling means. The aqueous slurry of the waste may optionally include a limited quantity of fine carbonaceous reductant.

Filtration may be effected at about 1 500 p.s.i. whereby the compacted waste exhibits a most homogeneous and consistent agglomeration and particle bond tenacity whilst possessing a significant and consistent moisture content, typically 15%. The kiln may be stable, or rotatable about an inclined axis.

The cake fragments may conveniently be fed into the kiln at ambient temperature and because they possess a higher degree of moisture than has been achieved hitherto via other process routes using an agglomerated feedstock the temperature of the upper end of the kiln is cooled by the additional water so that its temperature is reduced. For example if the lower end of the kiln is at a temperature of 1200 then the upper end may typically be 400"C-500"C.

The explosion hazard may therefore be contained, both by this measure and the maintenance of good reducing conditions within the kiln throughout calcination which results in acceptable levels of carbon monoxide carry over. It is because the gaseous products are at this low temperature that they may be presented to the recovery system (electrostatic precipitator, bag filter, venturi scrubber etc) without the need to provide extensive pre-conditioning to reduce the temperature as has been the case hitherto, particularly where after-burning has been necessary to dispose of the combustibles.

A further contributory factor is that there is less carry over of feedstock in the gaseous products which is primarily because of the very high strength possessed by the filter cake, and furthermore the moisture content of the cake is sufficiently constant to enhance process control of the reduction princess. In particular there is less variation in the amount of heat required to heat the filter cake feedstock and this enables steady conditions to be maintained in the kiln with greater ease; there are less excursions to conditions potentially generating combustible gases.Fewer fines in the kiln-by reason of the greater resistance to abrasion possessed by the 'strong' cake means a much lesser accretion formation on the sponge iron discharged from the lower end so that a higher quality product is obtained from both ends of the kiln.

The reductant may be solid, e.g. coke and/or coal, or even a fluid reductant may be used, and the combustion air may be injected at the lower end together with fuel oil, the gaseous products of combustion transported through the kiln, (primarily identified as Waelz oxide, which is rich in lead and zinc) issuing through the upper end into a drop-out chamber in which the heavier particles are removed from the gas stream. A significant proportion of this is the 'Waelz oxide' but a higher grade of this material is obtained from the primary recovery system, that is, the electrostatic pre cipitatorf wet scrubber or bag filter through which the gases then pass.

As mentioned therefore the Waelz oxide collection system is much simplified, there is not need to collect the coarse fraction for recycling as has been the case hitherto since the lesser degree of carry over means that there is no substantial coarse faction. There is thus no need for a cyclone-type recovery system or from form of pre-oxide collector other than the drop out chamber.

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 variety of metalliferous steel plant wastes e.g. arc furnace dusts in sludge or powder form, basic oxygen furnace waste slurries, millscale etc.

are dispensed at 1, 2, 3 into a slurry blending tank 4, make-up water being added to bring the water content to a desired level of about 60%.

The slurry from this tank is pumped, via a screen if necessary, into a main blending tank 5 which is continuously agitated and a quantity of powdered coal/coke 6 and if required limestone 7 is added at this stage to aid reduction and filter cake strength; alternatively any other binding agent, e.g. clay or fly ash may be introduced here. From here the slurry is filtered by a screen 8 where extraneous matter not passing through the screen is removed and dumped at 9, the sieved slurry being stored in a basin or sump 10.

this slurry is fed in a metered fahion to a bank of high pressure filters 11 which may conveniently be of the type depicted in Fig. 3 is U.K. Patent Application No. 37790/76.

Pressures up to 1 500 p.s.i. may readily be achieved in these filters.

In this instance the slurry is compressed to a pressure of about 1 500 p.s.i., the filtrate being recycled via loop 1 2 into the primary blending tank 4 whilst the compressed filter cake is discharged into a storage bin 1 3.

The filter cake is then continuously fed into a rotary kiln 14 together with the carbonaceous reductant 1 5a coal and coke mix. The kiln rotates slowly at between 0.5-1 r.p.m.

for example, the charge material migrating through the kiln with a typical residence time of about five hours in traversing the whole length, say 1 2 metres, this extensive residence time being aided by the provision of a dam ring at the lower end of the kiln. The kiln is fired by a gas or oil burner 1 6 and temperatures may typically be 1200 at the lower end and 400"C-500"C at the upper end, this latter comparatively low temperature being sufficient to dry the filter cake without causing breakdown by thermal shock.

The filtler cake issues from the lower end of the kiln as iron units in the reduced state, surplus fuel (char) and ash, and this material is then spray cooled by water as it passes through an inclined cooler 1 7. After drying in a dryer 1 8 it is then passed to a magnetic separator 1 9 from which non-magnetic char and ash is recovered and dumped at 20.

The sponge iron is then passed through a mesh screen 21 from which two fractions are recovered.

The combustion products issuing through the upper end of the kiln include volatiiized zinc and lead identified as 'Waelz oxide' together with various alkalis and careful control of the calcining conditions enables the CO levels to be kept below 10% in this area, 02 also being low-about 1%. The heavier components in this suspended matter drop out into the chamber 22 whilst the remaining combustion products are filtered in a bag filter 23-the gases passing through this filter may then be cleansed further by passage through an electrostatic precipitator 24, any residual zinc/lead compounds which might otherwise constitute a health hazard thus being recovered.

Very high grade products are obtained by the process according to this invention, the Waelz oxide being of much better quality than hitherto primarily because there is less carry over of feedstock in the gaseous flow and thus less degradation. In turn, the reason for this is that the filter cake is much stronger than agglomerates conventionally charged, their abrasion resistance is much higher and accordingly less fines are developed. In fact fewer fines are discharged than are charged since the iron units coalesce and the fines adhere to the reduced filter cake. Furthermore, an ash or char coating may 'glaze' the surface of the reduced filter cake, thus providing a useful corrosion inhibitor.

Because the temperature at the upper end of the kiln can be maintained at the comparatively low level of 400"C-500"C, the need for specific gas cooling means such as conditioning towers is avoided. The gases can be filtered direct since a bag filter will adequately withstand a temperature of the order of 200"C-250"C and a venturi scrubber 250"C-300"C, or an electrostatic precipitator (350"C) will withstand even higher temperatures, the required temperature drop simply being achieved during passage through the drop out chamber and associated ducting.

Substantial expense on conditioning equipment can be saved in this manner.

-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 this invention. For example, the carbonaceous additive coal/coke may not be added, if the carbon level in the waste slurry is sufficient there need be no further carbon added. The gaseous filter system shown may be modified, the electrostatic precipitator may be dispensed with, or alternatively an electrostatic precipitator or wet scrubber may be utilised instead of the bag filter. Various other screening and separation stages may also be adopted, and lower (or higher) filtration pressures than the typical value disclosed may readily be used consistent with obtaining the filter cake of the correct characteristics.

Further, it is to be understood that the reference to a kiln may, in addition to embracing shaft furnaces in general, embrace muffle furnaces ('ovens'), or hearth furnaces of the type including either a continuously movable 'merry-go-round' grate or a linearly movable grate.

Claims (11)

1. A process for obtaining metals from metalliferous wastes, wherein an aqueous slurry of said waste is subjected to high pressure filtration, in which the resultant compacted cake is charged into the upper end of a kiln where it is fired together with a carbonaceous reductant, whereby there is discharged from the lower end a sponge iron residue from which an oxide rich in lead and zinc has been volatilised and recovered from the upper end, the cake possessing a sufficiently high moisture content as to enable the upper end kiln temperature to be at a level low enough to permit gaseous products from the kiln to be filtered direct without the provision of any significant specific cooling means.

2. A process according to claim 1 in which a fine carbonaceous reductant and/or limestone is introduced into the aqueous slurry.

3. A process according to claim 1 or claim 2, in which filtration is effected at about 1 500 p.s.i, the moisture content of the cake being about 15%.

4. A process according to any one of claims 1 to 3, in which the kiln is rotatable about an inclined axis.

5. A process according to claim 4, in which axis the reductant is coal and/or coke and with this reductant and the filter cake are continuously fed into the upper end of the kiln.

6. A process according to claim 4 or claim 5, in which the kiln is fired by gas or oil burners at the lower end, the temperature in the latter area being about 1 200 C and the temperature at the upper end being about 400"C to 500"C.

7. A process according to any one of claims 1 to 6, in which the lead and zinc are recovered as Waelz oxide in bag filters, an electrostatic precipitator or venturi scrubber being provided as secondary filtration areas.

8. A process according to any one of claims 1 to 7, in which the residue discharged from the lower end of the kiln is spray-cooled in an inclined cooler, from which the sponge iron is extracted in a magnetic separator and recovered in different sized fractions by a mesh screen.

9. A process according to any one of claims 1 to 8, in which the high pressure filtration is effected in filters as described in Patent No 1557563

1 0. A process for obtaining metals from steel plant wastes substantially as herein described with reference to the accompanying drawing.

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