WO2000039351A1

WO2000039351A1 - Eaf dust treatment by pelletising and fluidised-bed reduction

Info

Publication number: WO2000039351A1
Application number: PCT/IB1999/002036
Authority: WO
Inventors: Peter William Ennis Blom
Original assignee: L & C Steinmuller (Africa) (Proprietary) Limited
Priority date: 1998-12-23
Filing date: 1999-12-22
Publication date: 2000-07-06
Also published as: AU1674700A

Abstract

The invention concerns a process for the treatment of EAF (electric arc furnace) dust. In the process preheated and decontaminated EAF dust is introduced into a fluidised bed reactor in which hematite is reduced by means of a hot reducing gas generated by reforming natural gas in a non-catalytic plasma-arc heating process. This produces an iron-rich material suitable for recycling to the electric arc furnace. The invention is characterised in that the EAF dust is pelletised prior to introduction into the fluidised bed reactor.

Description

EAF DUST TREATMENT BY PELLEΗSING AND FLUIDISED-BED REDUCTION

BACKGROUND TO THE TNVENTION

THIS invention relates to the treatment of electric arc furnace (EAF) dust.

WO 97/45564 (PCT/GB97/01445) describes a method of treating EAF dust in which preheated and decontaminated EAF dust is introduced into a fiuidised bed reactor in which hematite is reduced by means of a hot reducing gas generated by reforming natural gas in a non-catalytic plasma- arc heating process to produce an iron-rich material suitable for recycling to the electric arc furnace.

EAF dust is in very fine particulate form. In practice some 20% or more of the EAF dust may have a particle size less than 0,5 x 10^"6m. This can lead to problems in the fiuidised bed reduction reactor where the fine dust particles can cause blockages in the filter candles in the reactor, leading in turn to an increase in the pressure drop across the reactor. In addition, fine dust which is carried over in the exit gas stream can contaminate the zinc oxide fume product of the treatment process.

The present invention addresses this potential problem. SIJMMARY OF THE INVENTION

According to one aspect of the invention there is provided an EAF dust treatment process wherein preheated and decontaminated EAF dust is introduced into a fiuidised bed reactor in which hematite is reduced by means of a hot reducing gas generated by reforming natural gas in a non-catalytic plasma-arc heating process to produce an iron-rich material suitable for recycling to the electric arc furnace, characterised in that the EAF dust is pelletised prior to introduction into the fiuidised bed reactor.

If required a binder may also fed into the pelletiser for mixture with the EAF dust and water.

According to another aspect of the invention there is provided a method of pelletising EAF dust which comprises feeding predetermined quantities of EAF dust and water into a pelletizer which mixes the dust and water and forms the mixture into pellets, feeding pellets which are produced by the pelletiser into a drying kiln where they are dried at a first elevated temperature and feeding the dried pellets into a curing kiln where they are cured at a second, substantially higher temperature.

Other features of the invention are set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 (A and B) is a flow diagram illustrating the pelletisation and curing of EAF dust in accordance with the invention; and

Figure 2 shows a flow diagram of an EAF dust treatment process.

DESCRIPTION OF A PREFERRED EMBODIMENT

Figure 1 (A and B) illustrates the micro-pelletisation, drying and curing of EAF dust which will subsequently be fed to the fluidised bed reduction reactor in the EAF dust treatment process described in WO 97/45564, to which reference should be made for the details of that process.

In the drawing blended EAF dust 10 produced by an electric arc furnace is fed from a dust surge bin into a dust scale vessel 12 by means of a screw conveyor 14. At the same time, pelletising water 16 is fed into a water scale vessel 18 and a suitable binder 20 is fed into a binder scale vessel 22. The EAF dust, water and binder are then fed from the scale vessels 12, 18 and 22 to a pelletiser 24 in predetermined quantities.

It has been ascertained in initial experimentation that water additions of up to about 13% by weight and binder additions of about 1 to 3% by weight, depending on the type of binder used, are required for the pelletiser to produce acceptable quality green pellets.

The pelletiser 24 is a high intensity mixer consisting of a rotary drum and an agitator driven by separate motors. The drum and agitator rotate in opposite directions. Their rotation is continuous, even during charging and

SUBSTITUTE SHEET (RULE 26} discharging operations, and their rotary speeds are independently adjustable to achieve appropriate control over the size of the pellets which are produced.

The pellets produced by the pelletiser are discharged through hydraulically operated doors 26 onto a variable speed green pellet conveyor 28. During discharge of pellets, the belt is driven fast to spread the pellets evenly over its length. When the entire batch of pellets has been discharged from the pelletiser, the conveyor is switched to a slower speed to ensure a continuous feed of pellets to the pellet dryer furnace or drying kiln 30. It will accordingly be understood that while the pelletising steps described above are carried out batchwise, the feed to the drying kiln 30 is continuous.

A chute 32 feeds the green EAF dust pellets to the inlet of the drying kiln 30 at a constant rate. At this stage, the EAF dust pellets will typically contain approximately 12% water by weight. In the kiln 30, the entire water content is evaporated and the pellets are heated to a temperature in the range 120°C and 150°C. Heat is supplied to the drying kiln by the burner offgas from the curing kiln described below. The temperature of the incoming gas 34 is constantly controlled by a thermocouple control system to a value of 600°C maximum. If required, a control damper 36 adds dilution air before the hot offgas reaches the kiln 30.

Provision is also made for a direct-fired start-up or supplementary burner at the outlet of the drying kiln 30 to be used at start-up or in the event that the temperature in the kiln drops below a predetermined value. In the latter case, the burner will be fired automatically under control of the thermocouple control system on detection of the low temperature condition. The dried EAF dust pellets exit the drying kiln, at a temperature in the range 120°C to 150°C, onto a screen 38 with a ±2mm cut. The oversize material flows to a roller crusher 40 which reduces its size to an acceptable value, whereafter it rejoins the undersize material to be fed through a chute 42 to a curing furnace or kiln 44.

The curing kiln 44 is heated indirectly by burners located in the sidewalls of the combustion chamber. The burners are controlled at a selectable, preprogrammed temperature setpoint by thermocouples located in the roof of the combustion chamber. The EAF dust pellets are maintained at a temperature of about 950 °C for about 30 minutes in the curing kiln to obtain a sufficiently strong bond between the EAF dust and the binder. Process gas 46 enters the upstream end of the curing kiln and is extracted with offgas 48 at the downstream end.

Cured EAF dust pellets drop out of the curing kiln 44, into a hot drop-out box (not shown) though a slot in the circumference of the kiln tube and are transferred from the hot drop-out box for processing as indicated by the line 50.

Preferably both the drying and curing kilns have variable speed drives to enable their rotation to be slowed down when maintenance on downstream processing equipment has to be carried out.

The pelletising process and subsequent drying and curing are carried out so as to produce micro-pellets of EAF dust which preferably have a size in the range 0,5mm to 2mm. The process produces hard micro-pellets which will suffer minimum abrasive degradation when they are subsequently subjected to fluidise bed processing. In initial experimentation pellets formed without a binder as well as pellets formed with a Peridure binder and a Bentonite binder were tested. The results showed that, on induration in the curing kiln, the binderless pellets decreased quite substantially in size while Peridure bonded pellets also showed a size decrease but not to the same extent. The Bentonite bonded pellets showed little change. On subsequent fluidised bed treatment the average size increased for the binderless pellets but decreased for the Peridure and Bentonite bonded pellets. These results may be seen to indicate that the unbonded pellets offer lower resistance to decrepitation at the elevated temperatures prevailing during induration, i.e. curing, than the bonded materials, while during fluidised bed reduction, the unbonded pellets have a greater caking or adhesion tendency than the bonded pellets.

In addition cold strength tests were carried out for all three types of pellets using a ball-pan hardness test indicative of crushing strength. These tests showed that all three materials exhibited greater strength after curing than prior to curing. It was also found that the cured pellets exhibited good decrepitation resistance when subjected to fluidised bed reduction treatment.

The EAF dust treatment process described in WO 97/45564 is largely illustrated in Figure 1 (A and B). Figure 1 (A and B) also shows the pre- pelletising and curing steps, described above, which are carried out initially. It will be appreciated that both induration of the dust pellets and oxidation of the dust is carried out in the curing kiln 44, and that the remaining steps in the EAF dust treatment method described in WO 97/45564 are unchanged.

The curing kiln may be a rotary hearth or chain grate furnace.

It is believed that the pre-pelletisation and curing of the dust will lead to several advantages. Firstly, given the hardness of the pellets it is anticipated

SUBSTΓTUTE SHEET (RULE ) that there will be only a small percentage of fines in the fluidised bed reduction reactor which will make it possible to operate the system without an internal metallic filter system which is prone to blockage when substantial quantities of fines are present. Secondly, because the fluidised bed reduction reactor produces minimal fines the zinc oxide fume, which is a valuable product of the process, will be less prone to undesirable contamination. Thirdly the process will produce direct reduced iron in micro-pellet as opposed to powder form. The micro-pellet form which is produced is considered to be a superior product.

Claims

1.

An EAF dust treatment process wherein preheated and decontaminated EAF dust is introduced into a fluidised bed reactor in which hematite is reduced by means of a hot reducing gas generated by reforming natural gas in a non- catalytic plasma-arc heating process to produce an iron-rich material suitable for recycling to the electric arc furnace, characterised in that the EAF dust is pelletised prior to introduction into the fluidised bed reactor.

2.

An EAF dust treatment process according to claim 1 wherein predetermined quantities of EAF dust and water are fed into a pelletizer which mixes the dust and water and forms the mixture into pellets, pellets produced by the pelletiser are fed into a drying kiln where they are dried at a first elevated temperature and the dried pellets are fed into a curing kiln where they are cured at a second, substantially higher temperature.

3.

An EAF dust treatment process according to claim 2 wherein a binder is also fed into the pelletiser for mixture with the EAF dust and water.

4.

An EAF dust treatment process according to either one of claims 2 or 3 wherein the pellets are heated in the drying kiln to a temperature in the range 120°C to 150°C.

5.

An EAF dust treatment process according to any one of claims 2 to 4 wherein the pellets are maintained at a temperature of about 950°C for about thirty minutes in the curing kiln.

6.

An EAF dust treatment process according to any one of the preceding claims wherein, after drying in the drying kiln, EAF dust pellets are screened to a maximum size of 2mm.

7.

An EAF dust treatment process according to any one of the preceding claims wherein the EAF dust is formed into pellets having a size in the range 0,5mm to 2mm.

An EAF dust treatment process according to any one of the preceding claims wherein the iron-rich material is direct reduced iron in pellet form.

9.

A method of pelletising EAF dust which comprises feeding predetermined quantities of EAF dust and water into a pelletizer which mixes the dust and water and forms the mixture into pellets, feeding pellets which are produced by the pelletiser into a drying kiln where they are dried at a first elevated temperature and feeding the dried pellets into a curing kiln where they are cured at a second, substantially higher temperature. A method according to claim 9 wherein a binder is also fed into the pelletiser for mixture with the EAF dust and water.

11.

A method according to claim 10 wherein the binder is Peridure binder or

Bentonite.

12.

A method according to any one of claims 9 or 11 wherein the drying kiln is operated at a temperature in the range 120°C to 150°C.

13.

A method according to any one of claims 9 to 12 wherein the pellets are maintained at a temperature of about 950°C for about thirty minutes in the curing kiln.

14.

A method according to any one of claims 9 to 13 wherein, after drying in the drying kiln, EAF dust pellets are screened to a maximum size of 2mm.

15. A method according to any one of claims 9 to 14 wherein the EAF dust is formed into pellets having a size in the range 0,5mm to 2mm.