AU2002233406B2

AU2002233406B2 - Method for pelletization of iron ore

Info

Publication number: AU2002233406B2
Application number: AU2002233406A
Authority: AU
Inventors: Marc Bremont; Robert C. Moe
Original assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Cliffs Mining Services Co
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Cliffs Mining Services Co
Priority date: 2001-01-10
Filing date: 2002-01-09
Publication date: 2004-10-21
Anticipated expiration: 2022-01-09
Also published as: CA2433853A1; FR2819267B1; WO2002055745A1; BR0206399A; FR2819267A1

Description

WO 02/055745 PCT/FR02/00063 METHOD FOR PELLETIZATION OF IRON ORE A subject matter of the present invention is a process for the pelletization of iron ore and a plant for implementing said process.

The iron ore used for the manufacture of steel is very rarely available naturally in a form which can be used directly by the iron and steel industry. It is generally necessary to subject it to various stages of concentration and of preparation in order to render it compatible with current metallurgical processes. This is because ore of the correct grade is increasingly rare and the ore does not always meet the requirements of processes for the production of the metal.

The first conventional stage for the preparation of iron ore consists in giving it a specific chemical composition. This is because the ore is generally too poor in iron and too rich in undesirable oxides, such as silica, to be used in a metal production process. To improve its qualities, it is milled and then the iron oxide particles are concentrated until a high iron content is obtained. The second stage consists in giving this milled ore mechanical and physical characteristics which confer on it sufficient strength to withstand the stresses undergone in metallurgical processes, such as blast furnaces or directreduction furnaces. These characteristics are in particular mechanical strength at high temperatures, reducibility, resistance to abrasion, porosity, and the like.

This second stage passes through a phase of agglomeration of the ores in the form of particles with a medium particle size starting from the concentrated fines. One of these processes, known as "pelletization", makes it possible to obtain iron ore pellets with a diameter of the order of 10 mm. This conversion is generally 2 carried out by mining companies on the extraction sites or in the ports from which the ore is exported, the ore sold to iron and steel manufacturers then being an ore agglomerated in the form of pellets.

An example of a known pelleting plant has been represented in the appended figure 1. The plant comprises a first part 12 for formation of the crude pellets. The ore is mixed in desired proportions with certain additives, such as fluxes, binders and optionally a fuel in the form of coke dust, for example. Crude pellets exhibiting a suitable diameter but a low mechanical strength are thus present at the outlet 14 of the part 12 of the plant. The pellets are charged to a progressing conveyor (or screen) 16 forming a bed with a thickness of the order of approximately twenty centimeters. The screen 16 moves along in a tunnel in which is created a stream of hot gases obtained by burners (not represented in figure The screen 16 is composed of a belt, generally made of metallic materials, which is permeable to gases in order to allow forced circulation of hot gases through the bed of pellets via fans (not represented). The tunnel 20 is separated into several functional zones which can themselves be subdivided into several stages. First of all there is the drying zone 22, in which the free water present in the pellets is evaporated and then the water of crystallization is removed in its turn. The carbonaceous materials (fluxes) are calcined at the outlet of the drying zone, resulting in escape of carbon dioxide gas. In the following "preheating" section 24, the circulation of air in the bed of pellets is comparable with that in the dryer. The iron in the form Fe2+ present is oxidized by the oxygen present in the gases circulating through the bed of pellets while the bed of pellets passes through this section. This reaction gives off a significant amount of heat, which adds to that contributed by the burners. When the amount of Fe2+ present is insufficient, fossil fuel is mixed with 3 the ore and its combustion in this preheating section compensates for the contribution of heat given by the oxidation in the case where magnetite predominates in the pellets. These oxidation reactions of iron are very important for the quality of the product and must be complete before the sintering of the ore in order to obtain a final product of good quality. The sintering reactions begin at the end of the preheating section 24.

The pellets at the outlet of the section 24 are charged, in this embodiment, to a rotary furnace 26 equipped with a large burner. The hot gases move essentially transversely with respect to the direction of movement of the ore pellets. The pellets are heated by the flame and the radiation from the refractories from which the furnace is formed. They are thus maintained at a high temperature during their residence in the furnace. It is during this stage that the sintering and the recrystallization, which will give their mechanical properties to the pellets, take place. In other plants, the heating section can be composed of an extension of the tunnel analogous to that which defines the zones 22 and 24 (straight production line).

Finally, the plant is terminated by a cooling zone 28 in which the pellets are cooled to ambient temperature by circuits 30 for forced circulation of cold air through the bed of pellets. The air exiting from the cooler is generally used in the drying zone 22 or in the furnace 26.

If the productive output of the plant ics high and if the ore used comprises a high level of magnetite, the oxidation reaction does not have the time to take place properly in the preheating part 24 of the plant. As the oxidation does not take place in the rotary furnace, when the plant is equipped with it, the coolers complete this oxidation and consequently operate as 4 furnaces over a part of their length. As the fans are limited in capacity, the flow of air in the coolers cannot be increased. The remaining length for cooling the charge is no longer sufficient, which becomes a bottleneck with regard to the productive output of the plant. Furthermore, the sintering of a pellet which has not been completely oxidized results in a decline in the quality of the product obtained. It is commonly found that the oxidation has been carried out to only 60% at the outlet of the preheating. Consequently, it will be very useful both for the productive output and for the quality of the pellets to increase the degree of oxidation achieved at the outlet of the preheating without a reduction in productive output or even with an increase in the latter.

It should be noted that, in the case of a straight production line, that is to say without a rotary furnace, the problem of the oxidation in the coolers disappears. On the other hand, if a means of accelerating the oxidation during the preheating is found, this makes it possible all the same to increase the overall rate of the process and to improve the quality of the product by carrying out the oxidation before the sintering. Thus, whatever the type of plant, an improvement in the oxidation is desirable.

To accelerate the oxidation reaction, provision has been made to enrich in oxygen the air moving through the bed of pellets. It is this which is provided in particular in the documents GB 2 098 190 and US 4 313 757.

However, the amounts of air involved are high and the process operates with a large excess of air. The amounts of '02 to be employed to enrich the air are therefore enormous. Furthermore, as the equipment is not leaktight, there is a risk of a not insignificant portion of the oxygen used escaping frr6m the plant before having been used to produce the oxidation.

Although the use of oxygen appears to be technically desirable, the abovementioned techniques have been abandoned as, with these techniques, the use of oxygen results in a solution which is not economically viable because of the amounts involved.

The above discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.

One object of the present invention is to provide, a pelletizing process which makes it possible to improve the oxidation of the ore while limiting the consumption of oxygen necessary and thus while maintaining the economic viability of the plant.

To achieve this aim according to the invention, the iron ore pelletizing process comprises the following stages: pellets composed of a mixture of iron ore and of additives are provided, a bed of said pellets is formed, which bed moves along in a treatment plant comprising a drying zone, a preheating zone and a heating zone, said zones being fed by countercurrentwise circulation of hot gases produced in part by burners, and a cooling section positioned at the outlet of said zones; the temperature of said hot gases is measured in said zones or zone portions; said measured temperatures are compared with a predetermined maximum value and with a predetermined minimum value; and oxygen is injected, at least into some of said zones or zone portions for which the temperature measured is between said maximum value and said minimum value,. while controlling the flow rate of Document 6 the oxygen, whereby the oxidation of the iron ore present in said pellets at the outlet of said zones is substantially improved.

It is understood that, by virtue of the preliminary measurement of the temperature in the plant, it is possible to determine the zone portion or portions of the latter in which the temperature is between the maximum and minimum values and to limit the injection of oxygen to these zones or zone portions. These maximum and minimum values are respectively of the order of 1300 0 C and 6000C and more preferably 1100 0

C

and 850 0

C.

It has been demonstrated by the inventors that it is in this range of temperatures that the effect of the excess oxygen on the oxidation of the iron ore has the best output. It is understood that, by localizing the injection of an excess of oxygen, the consumption of this gas is limited while improving the performance of the pelletizing plant.

A second object of the invention is to provide a plant, in particular, for the implementation of the defined process.

The pelletizing plant for the treatment of ore pellets formed by ore and additives comprises: a drying zone, a preheating zone, a heating zone and a cooling section, means for moving along a bed of said pellets in said zones and in said section, said zones and said section being equipped with heating means and means for circulating hot gas; means for measuring the temperature in said zones or in portions of said zones, 7 means for comparing the measured temperatures with a predetermined maximum value and with a predetermined minimum value, means for feeding with oxygen, while controlling the flow rate thereof, at least some of said zones or zone portions for which the measured temperature is between said maximum temperature and said minimum temperature.

Other characteristics and advantages of the invention will become more clearly apparent on reading the description which follows of several embodiments of the invention given by way of nonlimiting examples. The description refers to the appended figures, in which: figure i, already described, shows a pelletizing plant according to the state of the art; figure 2 shows a pelletizing plant in accordance with the invention; figure 3 shows experimental curves giving the final degree of oxidation of the iron as a function of the maximum temperature for various oxygen contents; and figure 4 shows experimental curves giving the oxidation rate as a function of the maximum temperature.

As has already been described briefly, the pelletizing process consists in using a pelletizing plant of the same type as that already used but, in a first stage, the temperatures during the phase of normal operation of the plant in in particular the drying zone 22, the preheating zone 24 and optionally the heating zone 26 are determined. For this, each of these zones is preferably equipped with several temperature sensors, 8 such as C1, C2, C6, which are installed in the drying zone, in the preheating zone and optionally in the heating zone formed by the rotary furnace 26. These temperatures are supplied to a processing installation 40 in which the temperatures measured by the various sensors Ci are compared with the predetermined maximum and minimum temperatures corresponding to the temperature regions most favorable for producing oxidation by an excess of oxygen.

As has already been indicated, the temperature ranges are from 600 to 1300 0 C and preferably from 850 to 1100 0 C, the latter temperature range corresponding to the great majority of the ores to be treated.

Figures 3 and 4 show curves recorded during laboratory isothermal tests. A sample of ore is heated to a temperature T under an inert atmosphere. This temperature, once stabilized, is kept constant throughout the test.

The sample is then subjected to an atmosphere composed of nitrogen and of oxygen, the oxygen content of which is controlled. The variation in mass resulting from the oxidation of the ore by the oxygen of the atmosphere is recorded as a function of the time in order to quantitatively monitor the phenomenon.

The curves represented in figure 3 give the degree of final oxidation of the ore as a function of the maximum temperature under which the ore is placed for levels of injection of oxygen of 15% 18% 21% and 23% It is seen that these curves exhibit a maximum oxidation zone in the range from 700 0 C to 1300 0

C.

The curves in figure 4 give the mean oxidation rate of the ore in mol.g l .s1 as a function of the maximum temperature for four oxygen contents. These curves report the "oxidation rate", which exhibits a maximum in the temperature zone ranging from 800 0 C to 1200 0

C.

9 The choice of the temperature ranges 6000C to 13000C and preferably from 850 0 C to 1100 0 C results from a compromise between the results corresponding to these two series of curves.

After this comparison stage, the zone portions in which this temperature range is achieved are determined, in which zones or zone portions the injection of oxygen has to be carried out. After this preliminary phase of acquisition of information, the zones or zone portions are equipped with oxygen injectors, such as Ii and 1i2, which,.-in the specific example shown in the figure, are positioned in the preheating zone 24. These injectors are connected to an oxygen source 44 via an appropriate system. Control of these valves with the analogous means makes it possible to adjust the oxygen flow rate according to 'additional data, such as the content of oxygen in the air, and the like, which are supplied by other sensors., 'such as KI, K2, and the like. The'data supplied by these sensors and transmitted to the processing array 40 makes it possible to adjust the oxygen flow rate in the various zones or zone portions of the pelletizing plant.

It is understood that, in the case -where the plant comprises a rotary furnace 26, the injection of oxygen raises specific-problems and this solution will only be adopted in specific cases. On the other hand, when the heating zone is an extension of the preheating zone, the injection of oxygen can be carried out in the heating zone if the temperatures of the gases in this zone are within the abovementioned ranges.

SThroughout the description and claims of this specification, the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, integers or process steps.

Claims

1. An iron ore pelletizing process comprising the following stages: -pellets composed of. a mixture of iron ore and of additives are provided, a bed of said. pellets is formed, which bed moves along in a treatment plant comprising a drying zone, a preheating zone and a heating zone, said zones being fed by circulation of hot gases produced in part by burners, and a cooling section positioned at the outlet of said zones; -the temperature of said hot gases is measured in said zones or zone portions;, said measured temperatures are compared with a maximum value predetermined according to the ore treated and with a minimum value predetermined according to the ore treated; and oxygen is injected, at least into some of said zones or zone portions for which the tempera- ture measured is between said maximum value and said minimum value, whereby the oxidation of the iron ore present in said pellets at the outlet of said zones is substantially improved.

2. The pelletizing process as claimed in claim 1, wherein said maximum temperature is equal to 13000C and said minimum temperature is equal to 6000C.

3. The pelletizing process as claimed in claim 1, wherein said maximum 'temperature is equal to 11000C and the minimum temperature is equal to 8500C. 11

4. A pelletizing plant for the treatment of ore pellets composed of ore and additives, comprising: a drying zone, a preheating zone, a heating zone and a cooling section, means for moving along a bed of said pellets in said zones and in said section, said zones and said section being.equipped with heating means and means for circulating hot gases, means for measuring the temperature in said zones or in portions of said zones, means for comparing the measured temperatures with a predetermined maximum value and with a predetermined minimum value, means for feeding with oxygen, while control- ling the flow rate thereof, at least some of said zones or zone portions for which the measured temperature is between said maximum temperature and said minimum temperature.

The plant as claimed in claim 4, wherein said maximum temperature is equal to 13000C and said minimum temperature is equal to 600 0 C.

6. The plant as claimed in claim 4, wherein said maximum temperature is equal to 1100°C and said minimum temperature is equal to 8500C.

7. A process according to claim 1, substantially as hereinbefore described with reference to any of figures 2 to 4.

8. A pelletizing plant according to claim 4, substantially as hereinbefore described with reference to any of figures 2 to 4. DATED: 23 June 2003 PHILLIPS ORMONDE FITZPATRICK Attorneys for: L'Air Liquide, Societe Anonyme a Directoire et Conseil de Surveillance Pour L'Etude et I'Exploitation Des Procedes Georges Claude and Cliffs Mining Services Company