ES2248014T3 - IRON MINERAL SITERIZATION PROCEDURE WITH REDUCED TOXIC GAS EMISSIONS BY RESIDUAL GAS RECIRCULATION. - Google Patents

Abstract

A method of sintering iron ore in a sinter strand comprises providing a sinter feedstock and depositing said feedstock onto a travelling grate, igniting said feedstock using a sinter strand burner(s), and combusting said feedstock by drawing air through the feedstock with a plurality of wind boxes situated along the length of the strand, to produce sintered products rich in iron and waste gases. Waste gases from only those wind boxes whose temperature exceeds a predetermined level (200-250 DEG C) are recycled to provide combustion feed air for the feedstock or air for the sinter strand ignition burner(s), thereby substantially reducing or eliminating the production and/or emission of dioxins. <IMAGE>

Description

Sintering procedure for ore iron with reduced emissions of toxic gases by recirculation of waste gas.

This invention relates to the elimination of toxic by-products that are generated during sintering of iron ore In particular, the invention concerns a method and an apparatus to eliminate the synthesis of organic pollutants chlorinated, especially dioxins and polychlorinated furins (as far as briefly abbreviated PCDD / F), and gas evolution acids during sintering.

The conversion of iron ore, for example, into iron-rich agglomerates for later reduction in blast furnaces is a process known as sintering. The sintering process involves heating a layer of iron ore to produce its partial fusion that causes the individual iron particles to fuse together. The heat needed to achieve this is provided by the forced combustion of fine coke (known as coke coal) that mixes inter alia with the iron ore to be reduced and melts before being supplied to a sintering plant. The process is continuous; a mobile grill transports the sintered mixture, in the form of a bed, to an ignition cap that burns the coke on the upper surface of the sinter bed. Then, the combustion of the coke is maintained under the mixture by air flowing through the bed. The air circulation can be generated by a fan. Typically, the mobile grill carries the mixture to be sintered on a series of "air suction boxes" that suck air through the mixture to be sintered.

In a conventional sintering train, the waste gases pass from the air suction boxes, through suction tubes, to a "main air duct". These gases contain harmful emissions that include, inter alia , PCDD / F, nitrogen oxides and other acidic compounds, such as hydrogen chloride and sulfur dioxide. Therefore, the waste gas is typically treated to be washed before being expelled into the atmosphere.

PCDD / F present two of the by-products potentially more harmful combustion process sintering PCDD / F are a family of organic compounds chlorinated that have the general formulas (A) and (B):

one

       \ vskip1.000000 \ baselineskip
    

2

       \ vskip1.000000 \ baselineskip
    

3

4

The degree of toxicity varies depending on the number and position of chlorine substitutions in the ring aromatic.

There is evidence to suggest that PCDD / F precursors are formed by pyrolysis and chlorination of the organic material that represents the most important combustion products, such as hydrogen chloride, carbon monoxide, water, hydrogen, ethylene and acetylene. Therefore, PCDD / F are synthesized de novo as a result of thermal reactions between precursor compounds. For example, the precursor pentachlorophenol (C) can react producing dioxin octachlorodioxine (D). After formation, some of the PCDD / F can recombine downstream producing a different set of compounds that may have greater or lesser toxicity. Some of the non-toxic isomers can be converted into toxic isomers and vice versa.

It is also claimed that the "footprint" of Toxic isomers is specific to its formation mechanism; by example, in combustion processes, furans are synthesized in greater extension that dioxins and pentachlorate furan is the contributor greater than total toxicity.

Growing knowledge of the long effects term on the environment for the detachment of such toxic compounds to the atmosphere has generated the need for produce a relatively inexpensive and effective mechanism to decrease chlorinated and acidic organic compounds in the form of by-products gaseous combustion processes.

In the past, many efforts have been devoted to reduce toxic organic gases released in emissions from furnaces for incineration of urban solid waste materials. The control technology applied most frequently involves the use of a spray drying absorber followed by a device control of particulate matter, most commonly a filter of cloth or activated carbon. This technology controls the emission of acid gases, such as hydrogen chloride, by providing a neutralization reaction with calcium carbonate (lime). The resulting solid materials (for example, CaCl2) are captured downstream of the lime injection site. It is assumed that PCDD / F soft drinks are captured by surface absorption by the filter tank Although this method reduces toxic emissions soda in incinerator plants to a limited extent, It requires a lot of capital investment and is a complex process. In addition, it significantly increases the mass of solid waste to eliminate and introduce the problem of waste disposal toxic chlorinated organic solids.

In the patent EP-A-371945 describes another method used to eliminate solid urban waste. This document describes a method to extract dioxins from a gas, a method that comprises contacting the gas with a liquid aerosol during a predetermined period of time. The particles present in the aerosol are electrically charged and subsequently separated from the gas with the help of a wet electrostatic precipitator or field electric.

Gaseous ammonia or the compounds that they give off ammonia, like alkanolamines, have shown a potential to simultaneously control multiple emissions pollutants of interest, such as dioxins, hydrogen chloride, sulfur dioxide and nitrogen oxides.

The authors of the present invention have described in EP-A-0875587 a method Efficient and economical to reduce toxic emissions, more especially the formation of PCDD / F, in a sintering train keeping the amount of gaseous ammonia low released to the atmosphere and without the need to introduce complex modifications to existing sintering equipment.

This method comprises the sequential steps of producing a sintering feedstock by mixing in a substantially homogeneous, inter alia , iron ore with 0.01 to 0.09% by weight of a solid compound that releases ammonia by thermal decomposition, depositing the quoted feed material on a mobile grill and burn it to produce sintered iron-rich products.

The patent BE-A-877216 describes an oven of sintering tape in which, in an ignition hood of mineral, air from a material cooler is used sintered to achieve savings in natural gas consumption. He mineral to agglomerate is fed on the tape of material to sintering that moves over a row of suction boxes of air, the first of which is covered by a hood of ignition.

The patent FR-A-2444720 describes a system of gas circulation for a tape of material to be sintered, in the that the gases collected in suction boxes located along the tape is purified and fed to the final cooling zone of the sintering plant.

The patent GB-A-707583 describes a process of calcination or sintering in which the waste gases extracted through a bed of sintered material they are divided into relatively hotter and colder portions of gases, recirculating the hottest gas portion through the bed of material to sinter.

The authors of the present invention have now discovered an alternative method of reducing toxic emissions with or without the addition of exogenous chemicals to the train of sintering

It is known that dioxins form at a temperature within the range of 200 to 400 ° C. The formation of Dioxins have been reduced in other industries ensuring that gases residuals that can produce dioxins cool below 250 ° C as soon as possible to avoid reaction times prolonged at this critical temperature range. However, in a sintering train the situation is more complex; the gas passes to through a compact bed of the mixture to be sintered and the gas temperature at the bottom of the sintering train when Enter each air suction box will be different depending of the position of the box along the train. Typically, in the first 70% of the air suction boxes the temperature of output is relatively stable (approximately 100 ° C). This temperature rises rapidly towards the end of the train, reaching peaks up to 400 ° C, and then falls slightly at the end of the train.

It has been found that there is a good relationship between the temperature of the gas at the sampling point and the Dioxin level measured in the gas. Figure 1 illustrates the relationship between the temperature of the waste gases and the box number of air intake along the sinter train. The Figure 2 illustrates the relationship between PCDD / F formation in waste gases and the number of the air suction box at Sinter train length.

Consequently, in one aspect the invention provides a method of sintering iron ore in a train of sintering, method comprising providing a material of feed to sinter and deposit said material of feed on a mobile grill, burn the aforementioned feeding material using a burner or train burners sintering and burn the aforementioned feed material by aspirating air through the feed material with a plurality of air suction boxes located along the train, for produce waste gases and sintered products rich in iron,

characterized in that only residual gases from the final 20% of the plurality of suction boxes of air in which the temperature of the gases exceeds 250 ° C is they recirculate to provide flue gas for the material of power or gas for the burner or ignition burners of the sintering train, to substantially reduce or eliminate the dioxin production, and in which the waste gases cool quickly at a temperature below 200 ° C.

It has been found that the recirculation of waste gases in this way consumes dioxins in front of the flame when dioxins pass through the mixture of sintering or in the burner.

The invention may involve cooling the gases residuals in one or more of the air suction boxes. In a embodiment, the temperature can be reduced by injecting a liquid or a gas (for example, gaseous ammonia) in the waste gases in each air suction box and / or in the waste gases during its passage after each air suction box. The liquid can be water and can be added, for example, in the form of steam. Preferably, the liquid comprises ammonia that can reduce more the level of dioxins in the gas. The flow rate at which you can enter the liquid can be determined by the flow rate at which air is aspirated in the air suction boxes.

Alternatively or in addition, the outer surface of each air suction box and / or the outer surface of any duct that comes out of each air intake box it can be cooled by supplying a fluid (liquid or gas), by for example, water, for example, in the form of steam.

Alternatively or in addition, the mobile grill of the Sintering train can be cooled, for example, with water, by example, in the form of steam.

In another embodiment, a changer can be used of heat to lower the temperature of waste gases.

The feed material to be sintered can include 0.01 to 0.09% by weight of a solid compound that gives off ammonia by thermal decomposition. This compound that gives off Ammonia can be mixed substantially homogeneously with iron ore in the feed material to be sintered.

The invention is described below only example mode, with reference to the accompanying schematic drawings in which:

Figure 1 illustrates the relationship between the waste gas temperature and box number of air aspiration along the sinter train, as has been described earlier in the text,

Figure 2 illustrates the relationship between the formation of PCDD / F in waste gases and case number of air intake along the sintering train, as described earlier in the text, and

Figure 3 illustrates a conventional train of sintering

With reference to figure 3, the train comprises a mobile grill 1 on which constituents are deposited premixed from a sinter bed from a hopper 2. The mixing of these constituents is done in a mixer 3. The mixed constituents include iron ore and fines of iron ore, calcined lime, coke coal and, optionally, measured amounts of compounds that give off solid ammonia, for example, in the form of granules.

The deposited bed of feed material to sintering passes under an ignition cap 4, which makes burn the coke charcoal, increasing and continuing the combustion by the circulation of large quantities of sucked air through the bed by a series of air suction boxes (no shown) along the train. Sintered ore leaves the bed at the end of the grill away from hopper 2 and passes through A series of treatment stages. The waste gases of the combustion leaves the oven through a chimney 5.

The illustrated train is typical of many trains conventional, the essential difference being that gases residuals that contain toxicity levels higher than desired limits are specifically recirculated or that gases residuals of a suitable temperature for dioxin formation Toxic cools quickly.

It should be appreciated that the above is simply illustrative of methods and apparatus according to the invention and that various modifications can be easily made without leaving the scope of the invention defined in the claims attached.

Claims (11)

1. A method of sintering iron ore in a sintering train, a method comprising providing a feedstock to sintering and depositing said feedstock on a mobile grill, burning said feedstock using a burner or burners of the sinterizer and burn the aforementioned feedstock by sucking air through the feedstock with a plurality of air suction boxes located along the train, to produce residual gases and sintered iron-rich products, characterized in that only residual gases from of the final 20% of the plurality of air suction boxes in which the temperature of the gases exceeds 250 ° C is recirculated to provide combustion gas for the feed material or gas for the burner or ignition burners of the sinter train, for substantially reduce or eliminate dioxin production, and in which the Waste gases rapidly cool to a temperature below 200 ° C. 2. A method according to claim 1, in which the rapid cooling of waste gases is carried out in an air suction box. 3. A method according to claim 2, in which the temperature of the waste gases is reduced by adding a liquid or a gas in an air suction box. 4. A method according to claim 1, in which the temperature of the waste gases is reduced by adding a liquid or gas to the waste gases after these leave each air intake box. 5. A method according to claim 3 or claim 4, wherein the liquid is water in the form of steam. 6. A method according to claim 3 or claim 4, wherein the gas is gaseous ammonia. 7. A method according to claim 3 or claim 4, wherein the liquid comprises ammonia. 8. A method according to any one of the claims 3 to 7, wherein the flow rate to which the liquid is determined by the flow rate at which air is drawn into the air suction boxes. 9. A method according to any one of the claims 3 to 7, wherein the outer surface of a box air suction and / or the outer surface of any duct coming out of an air suction box is cooled by contribution of a fluid. 10. A method according to any one of the preceding claims, wherein the material of sintered feed includes 0.01 to 0.09% by weight of a solid compound that releases ammonia by decomposition thermal 11. A method according to claim 10, in which the ammonia-emitting compound is mixed so substantially homogeneous with iron ore in the material of feed to sinter.