GB2042376A - Pelletising waste materials - Google Patents

Abstract

A process for producing pellets from materials, e.g. steelworks waste, arising in fine dust or slurried form, in which an aqueous slurry of these materials (10) is subjected to high pressure filtration (11), and in which the resultant compacted filter cake is dried (14) fragmented (15), re-moisturised (20) and pelletised (21). <IMAGE>

Description

SPECIFICATION Pelletising waste materials This invention relates to a process of producing pellets from fine materials arising in dust or slurried form, for example metalliferous wastes arising from steelworks, mineral products, waste from ore preparation etc.

From one aspect the present invention relates to a process of producing pellets from materials arising in fine dust or slurried form, wherein an aqueous slurry of said materials is subject to high pressure filtration, and in which the resultant compacted filter cake is dried, fragmented, re-moisturised and pelletised.

The fragmentation of the filter cake may be effected by crushing or milling etc; the moisturising may be effected simply by adding water and/or a binder.

Filtration may be effected within the range 200 to 1 500 p.s.i.

The slurry may primarily be ferruginous waste products such as in-plant fines from ironmaking/steelmaking processes e.g. processed fumes, or rolling mill waste, e.g. millscale, however coke or coal binders or additives may be added to aid the subsequent processing, e.g. coke or coal would aid reduction or, e.g. limestone may be added to adjust the basicity of the feed material as necessary.

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

Pellets produced in accordance with this invention possess a much higher strength than pellets produced by more conventional routes. A crushing strength of six kilograms has been achieved, following high pressure filtration at 700 p.s.i., which is four times higher than had been achieved using a dry mix method with the same materials without high pressure compaction.

The reason for the increased peliet strength lies both in the wet mixing (as opposed to dry mixing) and in the inherent strength of the filter cake being imparted to the pellet subsequently produced. The filter cake, even though it is crushed, is not broken down to the individual component particles. This can give a more favourable size distribution of particles for pelletising.

Wet mixing is much more rigorous than dry mixing. This enables individual particles to become more intimately mixed and brought into contact with each other, which in turn leads to strengthening effects in any formed agglomerate by virtue of particle packing, and/or by virtue of surface chemical attractive forces between particles.

Further, the high pressure filtration is of such a pressure to 'weld' individual particles together to produce large composite particles.

If these are of a diverse size distribution, then this aids pelletisation.

A wide variety of different-blends may readily be pelletised in accordance with this invention including those which are difficult to pelletise by other routes. For example, it has hitherto been difficult to pelletise a blend of 14% powdered coke and 86% electric arc furnace fume but in accordance with this invention such a blend may readily be 'wet mixed', filtered under high pressure, dried, and the broken filter cake pelletised.

A further attraction arises from wet mixing in so far as it is safer than dry mixing from the aspect of airborne dust hazards to the operatives.

The pellets derived may be processed further in any subsequent furnace treatment, e.g.

in kilns or shaft furnaces, or in muffle furnaces ('ovens') or in furnaces having a circularly or linearly movable grate or hearth.

One typical application involves reduction in an inclined rotatable kiln to derive so-called sponge iron, for e.g. an arc furnace burden, and to recover from the upper end metallic oxides from the volatilized combustion products. The stronger pellets provide a much greater resistance to abrasion in the kiln resulting in less carry over of feedstock in the gaseous products and thus less degradation in the recovered oxide. Fewer fines in the kiln 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, gas or other carbonaceous fuel, e.g. coal. The condensed fume carried by the gaseous products of combustion transported through the kiln, primarily identified as Waelz oxide, which is rich in lead and zinc, issue 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 an electrostatic precipitator, wet scrubber or bag filter through which the gases may then pass.

With this feedstock preparation technique the Waelz oxide collection system is much simplified, there is no need to collect the coarse fraction for re-cycling as has been the case hitherto since the lesser degree of carry over means that there is no substantial coarse fraction. There is thus no need for a cyclonetype recovery system or any form of pre-oxide collector other than the drop out chamber.

The discharged sponge iron, which is in granular form, is water cooled, magnetically separated to dispose of the char and then screened.

In order that the invention may be fully understood one embodiment thereof will now be described by way of example, with refer ence to the accompanying drawing in which: Figure 1 schematically illustrates the process route for producing pellets in accordance with this invention, and Figure 2 schematically illustrates a kiln reduction process route for deriving sponge iron and recovering metal-rich oxides.

Referring now to Fig. 1 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 filter 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. 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 filtered slurry being stored in a basin or sump 10.

This slurry is fed in a metered fashion to a bank of high pressure filters 11 which may conveniently be of the type depicted in Fig. 3 in U.K. Patent Application No. 37790/76 (Serial No. 1557563). 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 1000 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 bin 1 3. From here the cake is fed into a rotary drier 14 operating at a temperature of about 400"C, and then broken up in a hammer mill 1 5 and discharged through a weigh feeder 1 6 onto a conveyor 1 7 together with fine coke/coal breeze discharged from a weigh feeder 1 8 by which the carbon content of the mix is enhanced.

The composition is throughly mixed in a mixer 1 9 where water is added at 20 and it is then pelletized in a conventional disc pelletiser 21, the pellets typically being 12 mm or so in diameter and possessing a very high strength.

Recorded mean crushing strengths of 6 Kgms have been achieved with lower pressures of 700 p.s.i.

Referring now to Fig. 2 there is shown a particular processing plant for utilising the pellets in deriving sponge iron units and recovering metal oxides.

More particularly, the pellets are continuously fed into a rotary kiln 22 and 23A together with the carbonaceous reductant 2313-a 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 fuel burner 24 e.g. gas, oil or coal fired.

The pellets issue 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 25. After drying in a dryer 26 it is then passed to a magnetic separator 27 from which non-magnetic char and ash is recovered and dumped at 28.

The sponge iron is then passed through a mesh screen 29 from which two fractions are recovered.

The combustion products issuing through the upper end of the kiln include volatilized 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, O2 also being low-about 1 %. The heavier components in this suspended matter drop out into the chamber 30 whilst the remaining combustion products are filtered in a bag filter 31, as mentioned no other coarse fraction recovery system is necessary. The gases passing through this filter may then if required be cleansed further by passage through an electrostatic precipitator 32, any residual zinc/lead compounds which might otherwise constitute a health hazard thus being recovered.

Very high grade products are obtained by this process following pelletisation 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 pellets are much stronger than those 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 pellets. Furthermore, an ash or char coating may 'glaze' the surface of the reduced pellets, thus providing a useful corrosion inhibitor.

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 carbonanceous additive coal/ coke may be added at either one of instead of the two stages shown or indeed if the carbon level in the waste slurry is sufficient there need be no further carbon added. Other binders or blend additives may also be added at either stage or both stages of coal/coke addition. The gaseous filter system shown may be modified, the electrostatic precipitator may be dispensed with, or alternatively an electro static precipitator or wet scrubber may be utilised instead of a bag filter.Various other screening and separation stages may also be adopted, and lower (or higher) filtration pressures than the typical values disclosed may readily be used consistent with obtaining the stronger pellets.

CLAI MS 1. A process for producing pellets from materials arising in fine dust or slurried form, wherein an aqueous slurry of said materials is subjected to high pressure filtration, and in which the resultant compacted filter cake is dried, fragmented, re-moisturised and pelletised.

2. A process according to claim 1, in which filtration is effected at pressure of between 200 and 1 500 p.s.i.

3. A process according to claim 2, in which the high pressure filtration is effected in filters as described in Patent No. 1 557563.

4. A process according to any one of claims 1 to 3, 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.

5. A process according to claim 4, in which a fine carbonaceous reductant and/or limestone is introduced into the aqueous slurry.

6. A process according to any one of claims 1 to 5, in which the filter cake discharged from the filters is dried in rotary driers at a temperature of about 400"C and then fragmented in a hammer mill.

7. A process according to any one of claims 1 to 6, in which the fragmented dried cake is admixed with fine coal and/or coke before re-moisturising to enhance the carbon content.

8. A process according to any one of claims 1 to 7, in which the re-moisturised mix is pelletised in a disc pelletiser.

9. A process for producing pellets substantially as herein described with reference to the accompanying drawing.

1 0. A process according to any one of claims 1 to 9, in which the pellets are charged into a rotary kilm in which they are fired together with a carbonaceous reductant whereby to produce a metallic sponge iron residue from which lead and zinc have been volatilised and recovered.

11. Apparatus for performing a process according to any one of the preceding claims, substantially as herein described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. static precipitator or wet scrubber may be utilised instead of a bag filter. Various other screening and separation stages may also be adopted, and lower (or higher) filtration pressures than the typical values disclosed may readily be used consistent with obtaining the stronger pellets. CLAI MS

1. A process for producing pellets from materials arising in fine dust or slurried form, wherein an aqueous slurry of said materials is subjected to high pressure filtration, and in which the resultant compacted filter cake is dried, fragmented, re-moisturised and pelletised.

2. A process according to claim 1, in which filtration is effected at pressure of between 200 and 1 500 p.s.i.

3. A process according to claim 2, in which the high pressure filtration is effected in filters as described in Patent No. 1 557563.

4. A process according to any one of claims 1 to 3, 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.

5. A process according to claim 4, in which a fine carbonaceous reductant and/or limestone is introduced into the aqueous slurry.

6. A process according to any one of claims 1 to 5, in which the filter cake discharged from the filters is dried in rotary driers at a temperature of about 400"C and then fragmented in a hammer mill.

7. A process according to any one of claims 1 to 6, in which the fragmented dried cake is admixed with fine coal and/or coke before re-moisturising to enhance the carbon content.

8. A process according to any one of claims 1 to 7, in which the re-moisturised mix is pelletised in a disc pelletiser.

9. A process for producing pellets substantially as herein described with reference to the accompanying drawing.

1 0. A process according to any one of claims 1 to 9, in which the pellets are charged into a rotary kilm in which they are fired together with a carbonaceous reductant whereby to produce a metallic sponge iron residue from which lead and zinc have been volatilised and recovered.

11. Apparatus for performing a process according to any one of the preceding claims, substantially as herein described with reference to the accompanying drawing.