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Following the privatization of Altos Hornos de Mexico in November 1991 four areas were chosen to improve blast furnace performance and hot metal costs. These areas included improvement of pellet quality, start of oil-gas co-injection in the blast furnace, improved control of gas flow in the blast furnace and start of monitoring program to predict the remaining life of the hearth of the major blast furnace at AHMSA.

The efforts resulted in the following improvements in the first half of 1992:

- Production level increased to 2.4 ton/m3/24 hrs; moreover, the annual production record of blast furnace 5 (hearth diameter 11.2 m, 37 ft) set in 1992, was 1.639 mln tonnes.

- Coke rates decreased by 60 kg/tonne, half of which was due to process improvements and the other half to injection of oil.

- Silicon standard deviations decreased from 0.22% in 1991 to 0.20% in 1992.

- Hot metal costs decreased with 10.6%.

- The major reline of BF 5 can be postponed by at least 1.5 year.

Altos Hornos de Mexico, S.A. de C.V. (AHMSA) is the largest integrated Steel plant in Mexico. In November 1991 it was bought from the government by the GAN group (Grupo Acerero del Norte), who entered into an agreement with Hoogovens Technical Services, Netherlands for provision of assistance in quality improvement, operation and technology transfer.

AHMSA was founded in 1941 with a 4.0 M hearth diameter blast furnace and four Siemens Martin open hearth steelmaking furnaces. Presently, AHMSA is the largest integrated Steel plant in Mexico; AHMSA produced in 1992 2.4 mln tonnes of liquid Steel from which a complete product range including hot rolled coils, cold rolled coils, tinplate heavy profiles, light profiles, reinforcing bars and wire is made.

The hot metal production comes from two operating blast furnaces, BF4 with a hearth diameter of 7.5 m and BF5 11.2 meter (25 ft and 37 ft). The raw materials for the blast furnaces comes from two coke plants, a pellet plant and a sinter plant. AHMSA owns its ore and coal mines, which are located in the area. The AHMSA coal contains more than 13.5% ash and the coke ash amounts to 17% ash.

Following the privatization of the AHMSA Monclova Works at the end of 1991, four major opportunity areas were chosen to enhance performance and reduce costs in the hot metal production:

- Improvement of pellet quality;

- Start of co-injection of natural gas/oil in the BF 5;

- Improvement control of blast furnace gas flow;

- Hearth monitoring of BF 5.

The present paper deals with the operational improvements of the major blast furnace, BF 5, during the first half of 1992.

AHMSA Pellet Plant started in 1984; it was designed to produce 3.0 mln tonnes per year based on ore from its own mines sited at Hercules, Coahuila, some 285 km (180 miles) from the AHMSA Steel Plant in Monclova. To feed the pellet plant, a pipeline transport is being used. the pipeline is also connected with "La Perla" mine at a 40 km distance from Hercules. La Perla was shutdown in December 1991 because it had reached the end of its life.

The design basis for the pellet plant was to use concentrate from Hercules and La Perla mines at 67:33 ratio. However, due to operating problems in the grinding system and metallurgical changes in the concentrate quality (% SiO2, alkalis) the designed pellet quality and quantity were not obtained until 1992.

In preparing for the situation after closure of the La Perla mine in December 1991, in February 1991 a plan to improve pellet quality was developed and several studies were initiated using only concentrates from Hercules. the main goal was to improve the pellets high temperature properties for better blast furnace operation and decrease overall ironmaking costs. Laboratory tests were done, using various binders; these were bentonite, hydrated dolomite, hydrated lime and synthetic binders. the test results are presented in Table 1. The best results were achieved using hydrated lime addition to increase the lime/silica ratio to 1.0 - 1.2, and dolomite to reach MgO/SiO2 ratio of at least 0.35.

With the support of the new AHMSA management, from January 1992 hydrated lime was used instead of bentonite to manufacture pellets with a lime/silica ratio of 1.1.

The trend of monthly data from 1991-1992 for lime/silica ratio and MgO/SiO2 is shown in Figure No 1. the quality of the production pellets is presented in Table 2. Reducibility improved from 78% to 86%, swelling (30 min) from 53 to 24% and the Burghardt index reached at international level.

Productivity in the pellet plant was not affected; pellet costs are even slightly lower with the present method. The most important result was, that upon charging the improved pellets in the blast furnace, coke rates decreased by more than 30 kg/tonne.

The business-plan developed prior to the privatization of AHMSA was based on a production level of 3.1 mln tonnes of liquid steel. The required amount of coke was studied in detail, making maximum use of the possible auxiliary fuel injections i.e. natural gas, oil and coal. A summary of these studies is presented in Table 3.

The conclusions can be summarized as follows:

1.-Reductions in coke rate were strongly needed to avoid coke purchases (every ton of purchased coke costs AHMSA 145 dollars).

 

 

 

 

2.- Natural gas injection was not a good auxiliary fuel, because of price, availability and coke reduction potential.

 

3.- To restart oil injection was an obvious solution in the short term. However, because of the high sulfur content of the Mexican fuel oil available (4.0 - 4.5%), the oil injection also was limited to as much as 40-50 kg oil/THM.

 

4.- A coal injection system for B.F. 5 requires investment of 20-25 million dollars and was not a solution for the short term. Nevertheless, it seems to be a good solution for the long term.

 

As a first step, the operating gas injection system of BF 5 was replaced by a system for co-injection of natural gas and fuel oil.

BF 5 has a hearth diameter of 11.2 meter (37 ft), 28 tuyeres and a working volume of 2163 cubic meters (76000 ft3). It is a self supported furnace with Paul Wurth top, and the original design included a facility to inject fuel oil through the tuyeres. This vintage oil injection system was scheduled to be partially rebuilt. Therefore, a major effort was put into the study and development of the lance design and determination of operation limits, that were required to start use of the co-injection system.

The developed system was submitted to a series of trials using two tuyeres during March 1992 and brought into operation in April with 14 tuyeres using natural gas-oil and the remainder with natural gas only. During April, all the trials with gas-oil lances were finished and from the results the final lance design was selected: the concentric type lance, where the natural gas was injected through the outer ring and oil through its center, Figure 3.

Based on the experiences gained and information from the literature, the following starting points were used:

- The lance has to be centered.

- There is a critical distance between the tuyere nose and the lance tip; in Bf 5 the distance is around 10 cm (4").

 

- A lance diameter chosen to prevent carbonization; at 6 mm (1/4") diameter at least 300 kg oil per lance per hr.

 

- Accurate control of injectants

As soon as the injection rate increases, control of burden and burden distribution is vital. in practice we choose to keep the ore layer constant and as a first step reduced coke layers as soon as coke rates decreased, while charging level was kept constant.

Without any major problems, oil injection was increased to 35 kg/tonne and the replacement of coke was 1.05-1.10 kg/tonne. Further operating results are discussed below.

The operation results of BF 5 are presented in Figures 2, 4 and 5 and Table 4. The results can be summarized as follows:

- The actions taken have reduced coke rate by 60 kg/THM and increased production capacities. An annual production record was set in 1992 at 1.639 mln tonnes. production at full blast was 2.4 ton/m3/24 hrs.

- Stability of the process increased as demonstrated by a decrease in standard deviation of Silicon from 0.22% in 1991 to 0.20% in 1992.

- Sulfur input has increased from 4.5 kg/tonne in 1991 to 5.5 kg/tonne in 1992. However, probably due to the increased process stability, S-levels in hot metal increased less than was expected and are so far controllable.

- Ironmaking costs have been reduced in our major blast furnace by 10.6%. The major influences being the lower coke rate and coke costs.

The present control of blast furnace gas flow is based on a limited number of measurements; the thermocouples in the wall of the furnace and the temperatures of the gas over the diameter of the furnace above the burden. At the beginning of 1992 a technologist was appointed to control the BF-process on a daily basis. The major changes in the process control philosophy introduced were:

1.- More consistent maintenance of burden level;

2.- Introduction of automated charging of the furnace;

 

3.- Introduction of standard procedures for process control;

 

4.- Improvement of preparation of stops;

5.- Introduction of daily and weekly process evaluation and the improvement of process information required for these evaluations.

 

Subsequent process results demonstrated the improvements obtainable with the new process control philosophy. for example, there are predictable changes in the burden distribution available, when the proportion of sinter changes. An example is given in Table 5.

BF 5 was revamped in 1985; the hearth was designed for 9.0 million tons of hot metal, using Nippon Electrode quality BC-5 and carbon blocks for walls and bottoms respectively, with 1500 m3/hour of external spraying cooling system and 750 m3/hour for the under hearth cooling system. Included in the original design were 24 thermocouples in the hearth bottom and wall.

In 1989, 1990 and 1991, the furnace stack was gunned using robot gunning, window gunning and traditional gunning respectively. In all three jobs, the burden was blown down to the tuyeres.

In the 1991 job, after 21 days of furnace shutdown, and 3.5 days after blown in, the hearth shell below tuyere 26 cracked, (3 m. long and 35 mm. wide).

Clearly, blast furnace 5 is at the end of its campaign. However, in order to operate as safely as possible a monitoring system was installed in order to track the refractory condition and to receive early warning of any imminent risks.

The major steps taken were:

1.- Revamping and expansion of the design thermocouple system with more than 60 additional thermocouples; introduction of computerized data collection. The former and present thermocouple configuration are presented in Figure 6.

 

2.- Hearth shell temperatures are being tracked on a daily basis.

3.- Core drilling samples were taken from the hearth zones where temperatures were higher than the average, especially below the iron notches.

 

4.- In addition to the core drilling results, mathematical model calculations were made as well by Nippon Electrode as by AHMSA.

 

On the basis of the measurements the process conditions of the blast furnace and the hearth thermocouple readings can be correlated; f.i. it became clear from the thermocouple readings, that water leakage of a tuyere leads to higher and unstable thermocouple readings.

the core drills came all more than 0.50 meter deep within the original refractories and during core drilling the higher refractory temperatures found, were around 300 ° C. From visual and chemical examination it was found, that no "brittle zone" was present in the samples.

The final conclusion of this effort is, that the hearth is in an unexpectedly good condition considering the fact that production on this moment already amounts to 120% of the design and that AHMSA has a history of high alkali-burdens. the reline scheduled for March 1992 has been postponed, as a first step to 1994.

The improvement of the blast furnace process was not limited to the major BF5 of AHMSA, but similar rends can be observed in the BF 4, which will be equipped with oil injection as well. With respect to future work, improvement programs have been prepared to increase injection rates, improve cast house operation and to increase process stability.

We greatly appreciate the contributions of Elmer Shigo and Maana Pattanayak during 1985-1991. The help pf prof. Dr. K van Laar with respect to the refractory condition inside BF 5 is gratefully acknowledged.

The present paper is the result of team work of management and operation at AHMSA. For this reason we dedicate the paper to our colleagues at AHMSA.

El Alto Horno opera actualmente a un ritmo de producción de 5,500 ton/día/Op, sus principales características se muestran en la figura 01 y se muestran en la tabla 1

El Alto Horno inicio su operación en el mes de Septiembre de 1994, el crisol tuvo un comportamiento normal y típico posterior al de una reparación mayor, pero en el periodo de1996 a 1999 los niveles de producción del alto horno fueron de alta productividad alcanzando 6200 Tons. por día.

Las practicas de vaciado desde el arranque del alto horno fueron desarrolladas con el sistema de taladro con martillo utilizando para terminar de abrir el agujero inyección de oxigeno mediante una lanza , posteriormente a partir del año 1996 y hasta el año 1999 estas practicas sufren un cambio técnico para la apertura de las piqueras cambiándolas al sistema de soaking bar con el objetivo de hacer mas eficiente el vaciado, cambio requerido de acuerdo a los niveles de alta productividad durante este periodo, este sistema presento una serie de ventajas y desventajas sobre el sistema convencional, tal como se muestra en la Tabla 03.

La velocidad de transferencia de calor entre el agua de enfriamiento y el crisol es un parámetro de vital importancia por lo que se rehabilitaron los sistemas para que permitieran un enfriamiento constante.

Como resultado de los puntos anteriores se ocasionó que las piqueras presentaran un flujo de gases que estaban emanando del interior del horno por la periferia de la chapa metálica, y por el interior del block de material vaciado, el cual estaba limitando el flujo de arrabio y escoria durante el vaciado del horno, así como promoviendo el incremento de carga térmica por el flujo de los gases calientes y por el bajo flujo de vaciado mermas de producción al tener que ajustar el proceso por presión diferencial.

Además como medida preventiva en el proceso de alto horno se inyecta mineral de titanio (TiO2) pulverizado vía toberas, este método se aplica con un sistema de arrastre neumático hasta las toberas a una distancia de tres toberas a cada lado de la piquera, y parando la operación de la piquera el mayor tiempo posible para generar un arrabio rico en titanio con el fin de formar una capa protectora de carburo de titanio en el área de alta temperatura de las piqueras, este seguimiento se lleva a cabo en forma diaria mediante un balance de materia y energía, con el fin de conocer la cantidad de titanio retenido en el interior del crisol, el mecanismo de formación del Ti(C,N) en la pared del crisol se puede ver en la Figura 09, en la figura 10 se puede ver el balance total de titanio durante cuatro periodos de inyección del mineral al crisol.

Como se puede apreciar en la Figura 11 el comportamiento de las temperaturas referenciadas al punto "D" de la piquera N°2 muestra un comportamiento que garantiza seguridad, ya que se logro bajar la temperatura de mas de 650 ° C. a nivel de 220 ° C., en la grafica se aprecian los principales acontecimientos que se llevaron a cabo en la periferia de la piquera para poner bajo control la temperatura.

Fundamentals of titanium-rich scaffold formation in the blast furance Herat. D. Bergsma and R.J. Freuehan. Corus Research Development Tech0nology.

Blast furnace Hearth design and operation for long campaign life. H. Stewart B. Titterington and E. T.James. British Steel plc. U.K.

Monitoring of hearth condition ( a novel experience) N.P. Singh, A. Mukhopadhay, G.P. Goswami. Bokaro Steel Plant Sail.

Prolongation of blast Furnace Campaigns Factors Affecting the Life of the hearth. E. Wilms, J. Janz, H. B. Lungen, M. Peters and P. Schmole Joint Meeting of AISI Iron Committee.

Monitoring and Maintenance of Blast Furnace Hearths, AHMSA experience. A. Garcia, R. Rodriguez, J. Borrego, F. Aldama, Y. Bañuelos. Altos Hornos de Mexico S.A.

Seminar on Blast Furnace Hearth Madhu G. Ranade, Fred Hyle, Roxie Graystone, Frank Fisher AISI Committee Ironmaking and Refractories.

The Tap Hole Zone – the critical Factor in Long Campaign Life. D. Jemeson, British Steel Research, M.G. Eden Kavaerner Metals, R. Gordon Kavaerner Metals.

UCAR Union Carbide Company, Refractory Systems.

The Management of Protective Hearth Skulls at British Steel Scunthorpe, Richard Hood, Senior metallurgist British Steel Scunthorpe Works.

Informacion Técnica del Alto Horno. A.H.M.S.A.

Certificate of Análisis and Compliance, Vesuvius USA.

 

 

CHART No.3

ALKALIES INPUTS

CHART No.4

ZINC INPUT

CONCLUSION

1.- The learning curve was fast; 12 months.

2.- The main problems for coal injection were: the wood in the coal and the problems in instrumentation and fluidization control systems at the GCI plant.