Juan A. López C., Raúl V. Ballesteros B.

Jesús M. Guipsot T., Rogelio Ramírez

Altos Hornos de México S. A. de C. V.

Monclova Coahuila, México

[Phone: (86) 49-36-00 Ext. 3614, Fax: (86) 33-76-59]

Based on pilot scale tests in a briquetting facility, Altos Hornos de México S. A. de C. V. (AHMSA) has developed a cold bonded Blast Furnace flue dust briquette to be consumed in BOF´s. Trials has been run in steelmaking shops and at the moment, briquette consumption is about 1000 Kg. per heat. The first results show that the use of this briquette do not modify the metallic charge requirements because its carbon content supply all the necessary heat for heat up, reduction and melting. This effect means an extra contribution of iron units into the metallic bath.

To increase competitive advantages, the Iron and steel industry is increasing its research trying to eliminate the waste oxides from their production process mainly due to the following reasons:

Environmental impact; more restrictions and less landfill availability.

 

Economical aspects: Recovery of valuable iron, carbon and flux units and decrease the cost by handling and disposal those materials.

 

Altos Hornos de México, S. A. de C. V., the main integrated steel plant in Mexico, since the privatization six years ago, has had a continuous growing in several areas and one of them is waste oxide recycling process.

The waste oxide materials are inevitable in the modern steelmaking and AHMSA as an integrated steel plant is not the exemption , of the total amount of consumed raw materials, around the 4 % is being handled at the end of the process as waste iron oxides.

The strategies to keep in control the waste oxide generation and its further recycling are:

In the process; operation practices to reduce its generation by means of improvements in Process Control. With the same Philosophy, using better handling systems that includes modifications on belt conveyors, discharge shutes, railroad handling and wagon dumping.

At the end of the process; as post- treatments.

After the process; recycling within the company or in external companies.

And finally disposal on prepared sites according to environmental regulations.

 

The last option represent the major cost. For this reason 100 % of waste oxides generation should be recycled or sell it for external applications.

This paper describes the blast furnaces flue dust briquettes development at AHMSA that includes since the beginning of briquetting plant operation, the different mixtures of binders and iron oxides, the consumption trials in the BOF and results.

The recycling route for the waste iron solids oxides at AHMSA directly depends of their Alkalis (K₂O + Na₂O) and Zinc content, plus the size distribution that is also involved, taking into account the present sinter plant operation with high percentage of concentrated Iron Ore (40 %) and considering this area as the main consumer of recycled waste oxides.

With this previous reference, several trials were performed trying to recover Iron and Carbon units in the waste oxides as much as possible, using the flue dust collected mainly in the environmental control equipment of the Blast Furnaces and the BOF´s. Besides the iron units recovery, an additional benefit is accomplished due to a better environmental control and the less cost involved in handling and landfill those materials.

This strategy was divided into several steps:

Quantify the flue dust generation at the Blast Furnaces.

 

Flue dust chemical and physical characterization, considering another feasible raw materials to perform the briquetting process, and projecting as possible their potential consumption in the Blast Furnaces and/or the BOF.

Under this consideration the materials involved were the blast furnace flue dust and the BOF sludge. The typical chemical and physical analysis are indicated in the table 1. Even the mill scale that is 100 % raw material for the sinter plant, was considered in this trials to increase the product iron content.

Based in the collected information, different mixtures were discussed and chosen to produce the dust briquettes using molasses and lime as additives. Figure 1 show the process flow chart.

The briquetting facility has a production capacity of 90 T/Day. The pocket rolls dimensions are 4.13 x 4.76 mm and the roll pressure is variable with a maximum of 190 Kg/cm². The standard operation pressure is 105 Kg/cm².

The waste oxides materials and additives were blended manually for the pilot scale test and fed directly to a Komarek briquetting press. Yield obtained was around 85 - 90 % and the fines generated were recycled to the process.

The briquettes produced in this way were stored within 2 - 3 days to reach enough curing time prior to be sent to the consumers. The results of these tests are show in the table 2.

After analyze this set of results, a preliminary conclusion was that briquettes produced were not good enough for the Blast Furnace consumption, because of the briquettes physical properties and on the other hand, the Zinc and Alkalis content but suitable for BOF process. As a consequence, a decision was made to produce certain amount to be used at the steel shops as a trial.

The actual briquette is being produced with 100 % BF flue dust with the additives mentioned in trial 11 on table 2. Table 3 show the chemical analysis where rogue elements for the blast furnace can be seen.

Minimum problems occurred with the briquettes production regarding proper control of the lime addition, molasses fluidity and the rolls wear.

The steelmaking facilities at AHMSA are Basic Oxygen furnaces with two casting technologies; ingot and continuous casting. This facilities are divided in two plants, plant No.1 with ingot route and plant No.2 with continuous casting route. The ingot route represent only around 30 % of the actual liquid steel production capacity (3.5 MTY).

*Test according the medium MICUM norm for coke.

The plant No.1 has a three BOF`s with a capacity of 80 T/Heat and two converters in plant No. 2 with a capacity of 150 T/Heat each one. In both steel shops the trials were carried out.

Before the industrial trials, some predictions were made using the static and dynamic model (SDM) for the BOF burden to evaluate the effects in steel chemical composition and the specific consumption of the metallic charge, fluxes and oxygen. This result was positive and with this basis a set of trials was initiated to confirm the predictions and define the proper way to do the briquette addition.

At the BOF facilities the briquette arrives to the underground hoppers by truck. At the beginning it was found an excessive degradation, due to the handling from the briquetting plant to the underground hoppers and the transportation to the charge and weighing hoppers level before the addition to the converter.

The briquette handling and transport degradation was 10 to 15 %, this degradation was affecting the process because the fines were passed directly to the sludge increasing the quantity and size particles of sludge generation.

In order to improve the process, some modifications were made in the manufacturing and transportation of the briquettes, having good results diminishing dust generation around 5 %.

There are different practices to add the briquette to the BOF; charging it in the scrap bucket, trough the fluxes charge system at different steps of the process or with specials feeders that means spending time by crane usage and operation delay time. The second option is possible when the fluxes charge system has enough capacity to handle this material like in the AHMSA situation. Defined this point, only the proper time of the process to add the briquette was tested.

The first trial of the blast furnace flue dust briquettes, was made at the BOF No.1 with the addition of up to 500 Kg/Heat of briquettes during the blow period, without affecting the process as it was predicted using the SDM.

To confirm this results an evaluate others parameters a second trials period was initiated at the steel shop No.2, here the addition was around 2.0 - 2.5 T/Heat. The addition practices was made in different ways; during the blowing time, with the fluxes addition and at the end of the heat for "hot" heats as coolant.

After these trials, in March of 1997, the addition of BF flue dust briquettes in the converters of the steel shop No.1 was initiated on a regular basis, beginning with the addition of 500 Kg/Heat up to 1000 Kg/Heat, it was found an increase of 0.0032 % in the steel sulfur content using the maximum briquette addition due mainly to the sulfur content in the coke.

According to the trials results it was found that the best moment for the addition was at the beginning of the blow, because the reaction and the smog generation were minimized, for that reason it was decided to add the briquettes together and during the fluxes addition.

The reaction that normally occurs is;

FeO _(Briquette) ---------- FeO _(Slag)

FeO_(Slag) + C_(Metal) ----- Fe_(Metal) + CO_(Gas)

However the briquette has carbon, for the reaction this carbon is taken instead of the carbon_(Metal). This reaction shows that the iron oxide reduction takes place in the slag, and from there, the iron goes to the metallic bath, so based on this results the briquette behave almost neutral, thermically speaking and do not modify considerably the metallic charge requirements. According to the SDM the hot metal increase 0.04 T/Heat and the scrap decrease in 0.56 T/Heat with an addition of 3 T/Heat of briquettes. So this means extra iron units at the metallic bath or the respective substitution by scrap.

Based on iron balances calculation over trials results, it appears that 75% of the iron content in the briquette reports to the steel. This calculations results was achieved during the industrial trials in both steel shops.

Regarding the effect on the chemical composition of the steel at the end of the blow, the briquette consumption do not shows any negative effects, however the metallic bath oxidation level is less with briquette than without it in around 100 oxygen PPM, this means no damages to the refractory lining and should represent less consumption of aluminum at the stirring station.

As coolant material the results has not been satisfactory yet, because the trials and results are not completely clear but it can be mentioned that the foam slag do not allow a good sampling practice.

The economic development in a number of countries indicates that by products in all different areas are more and more used as raw materials in various manufacturing branches. This implies considerable efforts to develop and optimize the utilization of those by products and as it was mentioned, there are two factors influencing the efficiency of the by products utilization; raw materials costs and the costs of disposal and environmental pollution.

In the steelmaking area, the recycling routes of the waste iron oxides are not always the same in all steel plants, because the different characteristics of the raw materials involved. Table 4, shows the slags and iron oxides generation in our plant with their recycling route and also their consumption in the operative departments.

Some of the recycling routes are completed as for instance the BOF slag fines with a 60 % of iron content, that are actually being recycled to the sinter plant. Others routes are in the implementation phase or just as a project.

An important industry who is a potential consumer of by products is a cement Company close to AHMSA, which is utilizing some part of our slags in its process. This cement plant normally use our BF slag and just began to consume inert BOF slag in order to use the iron oxides contained in the slag replacing the iron ore fines.

Currently the BOF sludge are not being processed in sinter plant because their high zinc content. Something similar occurs with the B. F. flue dust because of an excessive amount of alkalis. This is AHMSA´s main reason to choose the recycling route through the BOF, although at the beginning of 1998 a project to send the BOF sludge from the steelmaking shop No.2 to the pellet plant will be finished.

Another project under development is to recycle 100% BOF sludge to the oxygen converters using cement pellets Technology.

An important part that is now under research is the possibility to recycle the oily dusts and sludge generated at the rolling mills.

The AHMSA policy since its privatization in this field, is to use in site, recover or modify to sale any by product that represent some value or negative effects in the environmental control.

AHMSA had developed satisfactorily a flue dust BF briquette to be consumed at its BOF facilities.

More than 75 % of iron content in the briquette can be recovered by the metallic bath at BOF.

The briquette do not modify the metallic charge requirements because its carbon content supplied all the necessary energy to heat up, reduction and melting itself.

Briquette consumption do not represent negative effects on the chemical composition of the steel and do not observe damage in the refractory lining of the converter.

No emission problems resulted from the recycling trials.

Global recycling program by products at AHMSA is a continuos improvements policy.