Sponge iron tends to reoxidice , this reaction absorves oxigen from the atmosphere forming a solid oxide crust in the reduced particle, sometimes this crust acts as a difussion barriere that inhibits or reduces the reoxidation kinetics, depending on the material caracteristics and the storage design the particle increases its temperature to values that can reach the melting point. Even without melting, hot sponge iron reacts violently with water, generating ( water gas) sometimes when poured on top of ponds produces violent explosions. ■

Oversea transportation of DRI has been adressed by some suppliers , controled aereation, or passivation, aging in storage with multiple turnovers and fine material screeneeng plus carefull hold inerting and watertigthness are some of the established practices. time consuming, expensive and sometimes simply impossible. in large scale production.

in some cases automated systems with infrarred sensing and seppartion have been installed and operated succesfully.

runaway reoxidation fighting, would be another issue. ■

Some plants worldwide actually handle sponge iron fines without using an intentionally inert atmosphere. the usual way to handle the Product that comes from the reactor or multy hart furnace is by the use og travelling bins.

In some of this installations the originally sealed tarvelling bins discharge into originally seale4d bins that feed a melter fuirnace.

the seal valves are in most cases long gone , as the sponge reacts wit the air it heats up and consolidates promoting arching in the best cases, in most cases it consolidates to almost metalic density creating a solid plug.

This plug is removed by the use of oxigen torches, sledge hammers and occacionally explosives.

Solving this problem will be topic of another reply.

Regards

marco ■

Sponge iron has in general a small percentage of carbon , carbon reacts with air generating CO and CO2 gases that help keep the very fine dust aereated and free flowing.

When the carbon is exausted the dust bed colapses and sinterizes. Forming hard crusts.

the solution move it fast .. do not fill the open bins to the top .

Limit the exposed residence time or add some coal. ■

Yes.

It is in fact a good idea.

we did used lime or hdrated lime layer to handle pirophoric sponge iron during reclaim of a runaway reoxidation storage building some 30 years ago.

This was a 20,000 ton storage building with automated reclaim system. it protects the conveyor.

You have to see if the lime can block the IR wavelength of your IR sensing system. if you intend to separate the hot spots from the rest of the material.

Regards

marco ■

Indeed lime and more often bentonite , dolomite and even magnesite are used as coatings or as solid lubricants in handling and producing sponge iron.

Lime acts as a barriere , reducing the stickiness of the material by reducing the area of metalic contac. magnesium bearing materials as dolomite have been used in iron ore pellets for blast furnaces for many years now, to increase the melting point, and thus reduce the stickiness.

Every so often somebody reinvents the ligthbulb and rediscovers dolomite as a pellet additive or as a external coating, surprisingly ther often try to obtain patents and in some countries obtain patents in something that has been in the public domain for almost a century.

Some times management is convinced that this is a valuable trade secret, and try to enhance their sales by dressing them up with magnesite, or dolomite and try to sell a new and improved process, it takes all kinds to make a bussiness. ■

Sponge iron is produced in many ways , either in pellet form in bulk or in very fine particles, when stored in open silos , sponge iron will tend to stick to the walls , How to overcome this problem witjhout requiring sealing and inerting the bins?

In many cases a fine deposit of coal in the form of what is called free coal, is carried over by the product , in some other cases it is claimed that this coal is associated to the Iron in a coating of iron carbide.

Whatever the form this coal will react with the oxigen in the air producing CO2 this CO2 will be at termal equilibrium with the surrownding media, this will produce a expanded gas stream that will help keep the very fine particles in suspension.

will also provide a somewhat inert atmosphere.

When the carbon is depleted iron will take over reacting with air , producing a solid product Iron oxide , driving more air into the pile . and increasing the temperature , sometimes well bejond the melting point of the contained product. creating irom metheorites or solid crusts very difficult to remove without the use of power equipment.

the solution , move it fast , befoire the coal is exausted , or add more coal to your product , you might have some smoke , but it will give you more time.

be carefull with how you design your bins , a stagnant or slow moving volumen can create additional flow problems due to segregation. Remember the in bin residence time will be associated with the reaction/temperture profile and hence the stickiness.

Regards

marco ■

But what to do with a belt conveyor. That handles sponge iron and is experience transfer point fires .

the basic answer was for many years .

Stop the conveying and clean the transfer point.

But then you cannot use rock boxes?

Another solution was use water sprays ...

but then we will get water into our storage building or meltshop silos or ship holds?....

Then somebody tought of using inert gas.... and steam,.......

Finally somebody tried using a dust (dry) fire extingusher.

The temperature is high enough to dicompose the powder.

Pneumatic Lime injection and infrarred sensing is being tested at the moment. seems that they are having success.

Now some sponge irons are more reactive than others , or at least that has been claimed for years .

In any case your belt transfers have to be prepared for a dust fire , and your systen should be able to divert or to hold conveying without stopping production.

you must be carefull , redundancy in belt conveyors is expensive , and isometimes (ask H2 SIDOR) useless.

Redundant systems should be independent and the gates sytems should allow for paralel operation, otherwise there is no redundancy and a transfer point dust fire (frequent in sponge iron) can translate in sizable production loss. ■

Using water

The use of water to combat sponge iron fires .

In transfer points or elsewhere.

You can smother a sponge iron fire with water , if enough water is used , remember the product is hot iron , and it will react with water creating water gas , small explosions normally ocurr and you can see if the lighting conditions allow , small flames , and steam comming out of the piles of sponge iron.

Under normal consitions you will use water to protect the equipment , not really to try to cool the sponge iron , although is frequent to see peolple trying to doo this.

The sad facts are that even if you suceed to smother a sponge iron fire , you leavve the remaining load in conditios appropiate for a reignition to ocurr.

If you can dump the ingnited load into a furnace do it, and use the heat in your adventage . If you cant , make sure you have an open space to dump and extend or abandon the ignited or compromised product. if you have an infrarred sensing and sepparating station then set your sepparation point to somewhere between 90 and 150 ºc depending on your product and storage facilities , be carefull to do your homework first and define your runaway temperature .and hot spot size . (critical mass)

the ignited mass can be recycled later ( sometimes) as remet. If it was cooled withouth or with minimun sintering.

Regards

marco ■

USE OF WATER TO COMBAT TRANSFER POINT FIRES --

SOMEONE JUST SENT ME AN E MAIL ASKING FOR A DESIGN OF A SYSTEM TO EXTINGUISH A FIRE IN A TRANSFER POINT OF SPONGE IRON..

LETS ELABORATE ON THIS.

IF YOU HAVE A FLEXOWALL RUBBER CONVEYOR FEEDING A BIN, LET'S SAY AN ELECTRIC FURNACE FEEDING BIN WITH A RELATIVELY OPEN DISCHARGE AND NO INERTING, NOT EVEN A BOTTOM SEAL VALVE, BUT A BELT FEEDER . (NOT UNCOMMON BUT NOT WISE)...

One does not feed water on top of the conveyor, it will get in the bin and make things worse.

If you want to protect the conveyor, spray it from the bottom after the discharge , and keep the belt moving always.

If you think that the fire was started because the lime obverheated forget it.

There will be sponge iron fines fires with and without the lime overheating. What is more, if you feed the lime with the sponge iron you will redice the intensity of the dust fire and might even etinguish it , just by the dust dilution with the lime.

If you can screen the fines, just the very small fractions, less than a milimeter or even less than half a milimeter, that is the fraction that causes the fires .

If you are up to the challenge use a fluidized screen to separate only the very fine (fluidized with recirculated inert gas) .

Then you inject the fine fraction with your coal lances, or with a dedicated lance.

Regards

Marco ■

When combating an open storage building sponge iron fire, formally run away re-oxidation, there are some precautions you must take.

1. Believe it or not, the first one is not to allow anybody to fall asleep inside the storage building. The re-oxidation of sponge iron consumes the Oxygen inside the building very fast and people falling asleep can suffer anoxia.

2. If you have an automated reclaim system, use it. Reclam first the good product that is, protect what is still valuable. If you cannot do it:

3. Use an infra-red detection system with an appropriate separation gate. Set the separation values according to the reactivity of your product and the sensing area. I will be glad to send you an article about this. Let's say you set your separation point to 90ºC. Most products will be stable at 90ºC, but if your sensing area is too large, then product, above 150ºC, can go through and cost you problems. If you can, use a redundant two set points infra-red sensing system so that one will be checking the other. This system has been used successfully for over 25 years.

4. Protect your belts with fine lime, it will act as an insulator. Be careful not to deposit the lime on top of the sponge iron, it will block the infra-red signal.

5. Design your gates so that they can operate with a static column, that way if, for some reason, the flow stops, you can always divert to an alternative point.

6. Use redundat sensing stations and redundant gates after transfer, between sensing stations.

7. Reserve an isolated area for the hot product, it will be there for a long time. If you can feed it immediatly to the melt shop, better, but if, for any reason, it goes into a bin, it might get stuck in te bin and contaminate the rest of the product and have a melt-down inside the bin. So, use dedicated containers for the hot product. It is not uncommon to see hot piles to be watered-down. It is useless, but it helps people feel they are doing something.

8. If you can extend the hot product, good, it will burn faster and reach lower temperatures. Layers of 30 cm or less have been used in the past successfully, it takes a lot of effort to do this.

9. You can re-cycle the material that has been sensed and separated to the same storage building where you are reclaiming, just be careful to leave an empty space between the area that has been reclaimed and where you are re-cycling your product.

10. Be careful on the propagation of re-oxidation through the structure. Accumulated sponge iron dust deposited in the structural beams can act as a fire propagation channel. Clean them before you start reclaiming and re-cycling.

11. If your storage building has cells, good, keep an empty cell always to be able to re-cycle to it.

By the way, if you have a re-oxidation incident, don't only blame the smokers and go on your business as usual, do your homework, make sure you have a stable product (with stable product, I mean, a combination of storage design with product reactivity.

A set-up that will dissipate more heat than the one that is generated by the natural re-oxidation of fresh sponge iron. Sponge iron will always be reacting, the reaction rate will be diffusion limited as the products of reaction are solid and form a crust on top of the of the iron itself. Which means that it will react faster when it comes out of the reactor and slower after a few days.

A good transient storage is the one diseigned to dissipate more heat and limit the amount of air available for the reaction. The other extreme is also possible. Dissipate a lot of heat an allow air to react freely, this way you will increase the stability of the product faster, but you will lose some metallization.

Remember, when you move the sponge iron, some of this crust will break and it will re-heat again. Normally the reaction rate will be much slower.

If you run into trouble we will be glad to help you. Just drop us a line. (We do not manufacture, we only consult.)

If you want to learn how other people solve difficult problems, like handling sponge iron, or radioactive materials, root cause analysis, you can go to the TapRooT(R) World Summit in San Antonio, TX, in April. For details visit

http://www.taproot.com/summit.php

Regards,

Marco ■

I have rcecently been asked about the need for deludging a conveyor belt .

This means putting a lot of water on the belt. Well some processes can require such drastic measures. We tried this in a steel plant in australia, I believe with good results. The conveyor did not burn, the system was triggered by a heat sensor on the head pulley; the only draw back is that the water went into a bin that had hot material at the time and that was an undesirable situation.

Deludging a conveyor is sometimes required to start-up some plants, as they do not have a cooling circuit and the initial material in the reactor needs to be heated to a point where the appropiate reduction reactions take place. Before that point the material is not suitable for bricketting or the meltshop. This material used to be called remets for remetalization required. It was normally dumped to the ground and cooled by flooding it or allowing it to cool naturally. Sometimes it took several months to cool down.

If you have to flood your conveyors, you must remember that the poroduct in the belt has the heat capacity of iron, although much lower density, you kow the temperature and you know the rate, so you neede to remove that heat with water.

You set the minimun amount of water requred, now, all the water will not reach the material, as it willl act as a hot iro , evaporating and sometimes decomposing the water on the way; so you need to force the water in, in several stages, cooling the conveyor and then cooling the product. Ot would be best if you can spread the load to allow the water to reach the surface of the conveyor and the particles in one station. without loosing material to the sides of the belt.

Good luck.

Marco ■

just to put some color here is a foto af a storage building handling sponge iron on fire ,

the re are normally few flames , the flames are caused by a desperate move to suffocate the reoxidation using large cuantities of CO2 usles as it was , it gave us beautifull picture oportunities .

Attachments

storage fire (JPG)

■

Look out if you use infra red sensing to detect and isolate hot spots

Do not use the very cheap wide spectrum ones , select a large wave length one , so it can see trough CO2 and vapor, and is not affected by sunlight use more than one in each station Please, do not save cents here , this is your last point of defence , one single conflagration will make you loose from 1 to 10 million dollars

Use fast and reliable water or gas cooled equipment , make sure it reads the right temperature , if installed in a flexowall make sure it reaches a stable readout before the box pases .

use double set point controls , sepparte very hot from just hot sponge.

check yopur paticular storage -product ignition point heat generation vs heat dissipation. availavility of air .

Design your gates to operate even with static column of solids

Design your gates to operate with foreign material as steel bars or stones

Select your discharge points away froim your storage yards or isolate the area. ■