CONTINUOUS IMPROVEMENTS IN PELLETIZING, THE AHMSA EXPERIENCE

Altos Hornos de México S.A. de C.V.

* Norma Mayela Carreon

**Rodrigo Martinez Sanchez

*** Hector Rolando Mata Esparza

This paper describes the improvements in pellet production, quality, fuel consumption and plant availability at the AHMSA Pellet plant during the last 6 years, and its positive impact on Iron production and fuel consumption at the Blast Furnaces .

Major changes in process, equipment, human resources and most important issues involved in this process, such as development and production of self fluxed pellets, petroleum coke addition, development of special additives, mixtures consisting of hydrated lime, bentonite and dolomite on a regular basis, are well explained.

Altos Hornos de México S.A de C.V. (AHMSA) is an integrated Steel Plant, who was purchased from the Government in 1991 by Grupo Acerero del Norte (GAN).

It is situated in Monclova, State of Coahuila in Northern México, about 160 miles south of the US- México border.

The Pellet Plant was designed and constructed by DRAVO - LURGI with a straight grate and an original design capacity of 3.0 MMTPY.

2.1.- Reception and handling of concentrated Iron ore.

This facility includes :

· A Marcona Tank of 80,000 Metric Tons. capacity for the storage and reclaiming of concentrated Iron ore at a rate of 600 TPH.

· Two 150 Ft. diameter thickeners

· Three 50 Ft. Diameter storage tanks

2.2.- FILTERING SECTION

· 10 EIMCO Filters with 12 discs per

filter (9 ft Diameter Discs), each filter

with a total filtering area of 1320

square feet .

2.3.- BALLING AREA

· 6 balling discs of 24.60 ft diameter (7.5 m). Originally designed with five units, the sixth was added with the 3.55 MMT project.

2.4.- DOUBLE DECK GREEN BALL SCREENING

· This operation is done by means of an Abacus Hydraulics double deck roller screen with a design capacity of 1050 TPH.

· A lower deck of 39 rolls each 4¼ inch diamter, and an upper deck of 26 rolls, 25 of them are 4¼ inch diameter and one of 6 ½ inch.

2.5.- INDURATION OF GREEN PELLETS

· A Dravo-Lurgi straight grate pelletizing

machine of 464 m² effective area.

· 58 windboxes.

· Natural Gas as fuel.

· 44 burners.

. Process Gas Fans:

. Hood Extraction: 750 H.P.

. Windbox Extraction: 5000 H.P.

. Windbox Recuperation: 5000 H.P.

. Updraft Drying: 5000 H.P.

. Cooling Air : 5000 H.P.

2.6.- ADDITIVES

· Dolomite: 2 Bradley vertical roller mills

with 30 TPH capacity.

· Coke: a 12' 6" diameter ball mill

operating in closed circuit.

The process begins with the reception and handling area for concentrated Iron Ore, which is received trough a 238 miles length pipeline directly to the thickeners, and subsequently sent it to storage tanks where it will remains in suspension by means of agitators.

In this section, the Pet coke is added in the form of slurry, to be utilized as solid fuel, and to improve the Pellets physical quality.

From the storage tanks, an homogeneous pulp of orel and coke is then pumped to the filtering section.

This filter cake, is sent to storage bins and from these, to a conveyor belt where the additives dolomite, bentonite and hidrated lime, are mixed.

The material, previously blended and homogenized, is fed to the balling discs by means of variable speed conveyor belts that regulates the tonnage rate.

The green pellets are separated by size using the double deck screen, and are sent to the induration machine. The product is transported to a transfer station, and to the Blast Furnaces stock yards or the pellet silos.

Based on a continuous effort, mainly in maintenance, equipment upgrade, people training, and an excelent team work, the AHMSA Pellet Plant has increased its production by 15.3% over the original design capacity.

3.43 MMT had been reached during 1996, and for 1997, the bussiness plan goal is 3.55 MMT. A project to allow this production increase, was initiated in 1995. Major changes in equipment such as an upgrade of the existing balling discs, and the addition of a sixth balling disc has been finished.

Therefore, pellet plant productivity (Ton/ m²/day) has increased year by year from a design of 19.83 to 23.00 during 1996, as it is shown in charts number 1 and number 2.

4.1- IMPROVEMENTS IN QUALITY.

Prior to privatization, in May 1991, a project to increase pellet basicity (CaO/SiO₂) from 0.78 to 1.10 and the MgO content from 1.05 to 1.36 was initiated.

Pilot trials were performed during 1991, and during January 1992, 1 % of Hidrated Lime was added as a binder and gradually increased to 2.5%.

The pellets chemical quality is shown in figure No. 3, while table No. I shows the effect on the metallurgical properties, due to the changes in pellets chemistry .

It is clear that reducibility increases, along with a decrease in swelling and a better LTB.

The most important effects of the fluxed pellets at the pellet plan were:

decrease of induration temperature of about 20° C per each zone in the machine.

A decrease in natural gas consumption of 1.8 m³/ton pellet.

a decrease of green pellet reject of 5%.

Chart no.3

As a result, pellet phisical quality also improves as shown in chart No. 4. In this chart a decrease in pellet size due to the change to fluxed pellet, can be observed. The mentioned changes have worked out positively.

Chart no.4

4.2.- PETROLEUM COKE USAGE.

As an economic strategy, a practice to use

petroleum coke at the pellet plant, instead of own coke fines has been developed. Major differences between coke fines and pet coke are shown in table No. II. Table No.III shows the effect on the pellet quality. No negative effects on pellet or environment had been experienced. A lower Natural gas consumption and a higher productivity has been achieved.

Table No.II

Table no.III

4.3 QUALITY SYSTEM ISO - 9002.

In February 1996 the implementation of the system started which resulted in certification by the company SGS in October 1996.

The most relevant benefits are:

uniformity of criteria in the use of established procedures

development of personnel by training

better communication between client and supplier

programmed inspection and continuous supervision of the main process equipment

planning and development of corrective actions when deviations occur.

 

 

The increase of the basicity index and MgO content in the pellet led to better metallurgical properties. This added to the better coque quality has contributed to a decrease in coke rate and productivity improvement of the Blast Furnace as shown in fig. 5.

Chart no.5

It is necessary to mention that the coque rate in 1994 was high due to the fact that Blast Furnace#5 was down for a reline and the much smaller and less efficient Blast Furnace# 2 was in production.

The most important achievements during this continuous effort are :

Improvements in Chemical Quality through the development of self fluxed pellet

Increase in production capacity of 15.3 %, for 1996 plus 3 % scheduled for 1997.

Increase in iron production, where pellets are more than 75 % of the number 4 and 5 blast furnaces burden.

Use of solid fuels; fine coke as well as petroleum coke.

February 1996 brought the initiation for the implementation of the Quality System under the ISO Standard 9002.

The certification of the Quality System from SGS , was achieved in October of 1996.

Because of this success, a project to increase the self fluxed pellet production capacity to 3.8 MMTY without penalties in pellet metallurgical quality, has been set as a priority in AHMSA´s Business Plan for 1998, and was initiated in October 1996.

 

- Increase the basicity of the pellet to 1.4 and the MgO content to 1.7.

- Improve the quality of green pellets by a better inder usage

Increase the porcent + 3/8 - ½ ( target 50 % ) moving the balling discs angles

 

 

 

 

ACKNOLEDGEMENTS

 

Los autores del trabajo agradecen a todo el personal de la planta peletizadora así como al grupo directivo el apoyo brindado para lograr las metas establecidas y la realización del presente articulo.