Iron ore sintering process with reduced emissions of toxic gases by waste gas recirculation

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°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.

Description: This invention relates to the suppression of toxic byproducts which are generated during sintering of iron ore. In particular, the invention concerns a method of and apparatus for suppressing the synthesis of chloro-organic pollutants, especially polychlorinated dioxins and furins (hereinafter referred to as PCDD/Fs), and the release of acidic gases during sintering. The conversion of iron ore into for example iron rich agglomerates, for subsequent reduction in blast furnaces is a process known as sintering. The sinter process comprises heating a layer of iron ore until partial melting occurs to cause individual iron particles to become fused together. The heat necessary to achieve this is provided by forced combustion of fine coke (known as coke breeze) which is mixed inter alia with the iron ore to be reduced and fluxes prior to delivery to a sinter plant. The process is continuous; a travelling grate transports the sinter mix as a bed to an ignition hood which ignites the coke in the upper surface of the sinter bed. Thereafter, the combustion of the coke lower down in the mix is maintained by air flowing through the bed. The air flow may be generated by a suction fan. Typically, the travelling grate carries the sinter mix over a series of "wind boxes" which draw air through the sinter mix. In a conventional sinter strand, the waste gases pass from wind boxes through "wind legs" which link to a "wind main". These gases contain noxious emissions including inter alia PCDD/Fs, oxides of nitrogen and other acidic compounds such as hydrogen chloride and sulphur dioxide. Therefore, the waste gas is typically treated to clean the gas before being exhausted to the atmosphere. PCDD/Fs present two of the most potentially harmful combustion by-products of the sinter process. PCDD/Fs are a family of chlorinated organic compounds having a general formula (A) and (B) respectively: The degree of toxicity varies and depends on the number and position of chloride substitutions on the aromatic carbon ring. There is evidence to suggest that PCDD/F precursors are formed from the pyrolysis and chlorination of the organic material that represents the more primary combustion products such as hydrogen chloride, carbon monoxide, water, hydrogen, ethylene and acetylene. PCDD/Fs are thus synthesised de novo as a result of thermal reactions between precursor compounds. For example, the precursor pentachlorophenol (C) can interact to produce the dioxin octachlorodioxin (D). After formation, some of the PCDD/Fs may recombine downstream to produce a different set of compounds which may have higher or lower toxicity. Some of the non-toxic isomers may be converted to toxic isomers and vice versa. It is also claimed that the 'fingerprint' of toxic isomers is specific to their formation mechanism; for example in combustion processes, furans are synthesised to a greater extent than dioxins and the pentachlorinated furan is the major contributor to total toxicity. The increasing awareness of the long term deleterious effects on the environment by the release of such toxic compounds into the atmosphere, has generated a need to produce a relatively cheap and effective mechanism of decreasing the chloro-organic and acidic compounds released as gaseous by-products of combustible processes. In the past, much effort has been directed towards reducing the toxic organic gases released in stack emissions from incineration of municipal waste materials. The most frequently applied control technology involves the use of a spray dryer absorber followed by a particulate matter control device, most commonly a fabric or activated carbon filter. This technology controls the emission of acidic gases such as hydrogen chloride by providing a neutralising reaction with calcium carbonate (lime). The resulting solid materials (e.g.CaCl 2 ) are captured downstream of the lime injection site. PCDD/F gases are assumed to be captured by surface absorption by the filter deposit. Although this method reduces gaseous toxic emissions from incinerator plants to a limited extent, it is capital intensive and procedurally complex. In addition, it significantly increases the mass of solid waste to be disposed of and introduces the problem of toxic chloro-organic solid waste disposal. Another method used in municipal waste disposal is exemplified in EP-A-371945. This document discloses a method for extracting dioxins from a gas which comprises contacting the gas with a liquid aerosol over a predetermined period of time. The particles present in the aerosol are electrically charged and subsequently separated from the gas with the aid of a wet electrostatic precipitator or electric field. Ammonia gas or ammonia releasing compounds such as alkanolamines, have demonstrated a potential to control simultaneously emissions of several pollutants of concern, such as dioxins, hydrogen chloride, sulphur dioxide and oxides of nitrogen. The present inventors have disclosed in EP-A-0 875 587 an efficient and cost effective method of reducing toxic emissions, more especially PCDD/F formation, from a sinter strand, whilst keeping the quantity of ammonia gas released into the atmosphere at low levels and without the need to introduce complex modifications to existing strand equipment. Their method comprised the sequential steps of producing a sinter feedstock by substantially homogeneously mixing inter alia iron ore with 0.01 to 0.09% by weight of a solid compound which releases ammonia on thermal decomposition, depositing said feedstock onto moving grate and combusting the same to produce sintered products rich in iron. BE-A-877216 discloses a sinter belt furnace in which air leaving a sinter cooler is used in an ore ignition hood to achieve savings in natural gas consumption. Ore to be agglomerated is fed onto the sinter belt which travels above a row of wind boxes, the first of which is covered by an ignition hood. FR-A-2444720 discloses a gas circulation system for a sinter belt in which gases collected in suction boxes positioned alongside the belt is purified and fed into the final cooling zone of the sinter plant. GB-A-707583 discloses a calcining or sintering process in which waste gases drawn through a sinter bed are divided into relatively hot and cooler gas portions with the hotter gas portion being recycled through the sinter bed. The present inventors have now discovered an alternative method of reducing toxic emissions with or without the addition of exogenous chemicals to the sinter strand. Dioxins are known to be formed in the approximate temperature range 200 to 400°C. Dioxin formation has been reduced in other industries by ensuring that waste gases which may produce dioxins are cooled to below 250°C as soon as possible to avoid prolonged reaction times in this critical temperature range. However, in a sinter strand the situation is more complex; the gas passes through a packed bed of sinter mix and the gas temperature at the bottom of the sinter strand as it enters each wind box will be different depending on the position of the wind box along the strand. Typically, for the first 70% or so wind boxes, the exit temperature is relatively stable at approximately 100°C. This rapidly rises towards the end of the strand, peaks at up to 400°C and then drops slightly at the end of the strand. It has been found that a good correlation exists between the gas temperature at the sampling point and the dioxin level measured in the gas. Figure 1 illustrates the correlation between the temperature of the waste gases and the windleg number along the sinter strand. Figure 2 illustrates the correlation between the PCDD/F formation in the waste gases and the wind box number along the sinter strand. Accordingly, in one aspect the invention provides a method of sintering iron ore in a sinter strand comprising 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, characterised in that only waste gases from the final 20% of the plurality of wind boxes whose waste gas temperature exceeds 250°C are recycled either to provide combustion feed gas for the feedstock or gas for the sinter strand ignition burner(s) in order to reduce substantially or eliminate the production of dioxins, and wherein the waste gases are rapidly cooled to a temperature below 200°C. It has been found that recycling the waste gases in this way consumes the dioxins at the flame front as the dioxins pass through the sinter mix or at the burner. The invention may involve the cooling of the waste gases in one or more of the wind boxes. In one embodiment, the temperature may be reduced by injecting a liquid or a gas (e.g. gaseous ammonia) into the waste gases in the or each wind box, and/or into the waste gases in their passage subsequent to the or each wind box. The liquid may be water for example, the water may be added in the form of steam. Preferably, the liquid comprises ammonia which may further reduce the level of dioxins in the gas. The rate at which the liquid may be introduced may be determined by the rate at which air is drawn into the wind boxes. Alternatively or in addition, the external surface of the or each wind box and/or the external surface of any conduit leading from the or each wind box may be cooled by a supply of fluid, either liquid or gas, e.g. water, for example, in the form of steam. Alternatively or in addition, the travelling grate of the sinter strand may be cooled, for example, with water, e.g. steam. In another embodiment, a heat exchanger may be used to lower the temperature of the waste gases. The sinter feedstock may include 0.01 to 0.09% by weight of a solid compound which releases ammonia on thermal decomposition. The ammonia compound may be substantially homogeneously mixed with iron ore in the sinter feedstock. The invention will now be described by way of example only, with reference to the accompanying diagrammatic drawings in which:- Figure 1 illustrates the correlation between the temperature of the waste gases and the windleg number along the sinter strand as described in the text above; Figure 2 illustrates the correlation between the PCDD/F formation in the waste gases and the wind box number along the sinter strand as described in the text above; and, Figure 3 illustrates a conventional sinter strand. Referring now to Figure 3, the strand comprises a travelling grate 1, onto which pre-mixed constituents of a sinter bed are deposited via a hopper 2. Mixing of these constituents is effected in a blender 3. The mixed constituents include iron ore and iron ore fines, burnt lime, coke breeze and optionally measured quantities of ammonia releasing compounds in solid, e.g. pellet form. The deposited sinter bed feedstock passes below an ignition hood 4, which ignites the coke breeze, combustion being enhanced and continued by the flow of large quantities of air, drawn through the bed by a series of wind boxes (not shown) along the stack. Sintered ore leaves the bed at the end of the grate remote from the hopper 2 and passes through a series of treatment stages. Waste gases of combustion leave the furnace through more stacks 5. The illustrated strand is typical of many conventional strands, the essential difference being that the waste gases which contain levels of toxicity exceeding desired limits are specifically recycled or that the waste gases of a temperature suitable for the formation of toxic dioxins are rapidly cooled. It will be appreciated that the foregoing is merely exemplary of methods and apparatus in accordance with the invention and that various modifications can readily be made thereto without departing from the scope of the invention as set out in the appended claims.

Claims: A method of sintering iron ore in a sinter strand comprising 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, characterised in that only waste gases from the final 20% of the plurality of wind boxes whose waste gas temperature exceeds 250°C are recycled either to provide combustion feed gas for the feedstock or gas for the sinter strand ignition burner(s) in order to reduce substantially or eliminate the production of dioxins, and wherein the waste gases are rapidly cooled to a temperature below 200°C. A method as claimed in claim 1 wherein rapid cooling of the waste gases is effected in a wind box. A method as claimed in claim 2 wherein the temperature of the waste gases is reduced by injecting a liquid or a gas into a wind box. A method as claimed in claim 1 wherein the temperature of the waste gases is reduced by adding liquid or gas to the waste gases after they leave the or each wind box. A method as claimed in claim 3 or claim 4 wherein the liquid is water in the form of steam. A method as claimed in claim 3 or claim 4 wherein the gas is gaseous ammonia. A method as claimed in claim 3 or claim 4 wherein the liquid comprises ammonia. A method as claimed in any one of claims 3 to 7 wherein the rate at which the liquid is added is determined by the rate at which air is drawn into the wind boxes. A method as claimed in any one of claims 3 to 7 wherein the external surface of a wind box and/or the external surface of any conduit leading from a wind box is cooled by a supply of fluid. A method as claimed in any one of the preceding claims wherein the sinter feedstock includes 0.01 to 0.09% by weight of a solid compound which releases ammonia on thermal decomposition. A method as claimed in claim 10 wherein the ammonia compound is substantially homogeneously mixed with iron ore in the sinter feedstock.

Inventors: SOUTHERN STUART MARTYN, SOUTHERN, STUART MARTYN
Applicants: CORUS UK LTD, CORUS UK LIMITED
Application year: 2000
Main classification: C22B1
IPC codes: C22B1