FR2964392A1 - METHOD FOR THE VALORISATION OF DUST OF ELECTRIC STEEL - Google Patents

Abstract

The invention relates to a process for recovering dust from electric steelworks from second-stage electric steel furnace furnaces, in which part of the dust collected after melting in the electric furnace is agglomerated in the presence of a source of sodium oxide. and a carbon source for forming balls, the balls thus obtained being then introduced into said electric furnace with scrap metal.

Description

The present invention relates to the technical field of the treatment of dust from steelworks, in particular dust from electrical steelworks. STATE OF THE ART Electric steel mill dust is recovered by bag filters for capturing the ashes of the fumes of secondary melting furnaces of electric steel mills. These steelworks are generally supplied with recovered scrap, the composition of which reflects in particular that of automobile carcasses - composed of mild and stainless steels, chromed, phosphated, cadmium and / or electrozinc, alloyed, mixed with copper cables and to their connections, lead-acid batteries, plastics (PVC, PE, PP, PTFE, PVDF, etc.), or paints - as well as building waste - mild, galvanized, phosphated, mixed with mineral waste steels (concrete, silicoaluminous) or paints. Steel mill dust contains variable amounts of elements such as zinc, iron, lead, copper, manganese, chromium, cadmium, chlorine and fluorine. This variability depends on the quality of the scrap used. A typical steel mill dust composition can be illustrated as follows: Item Element by Weight at 40% by Weight 0.1 to 2 Mg Na 3 to 10 5 Fe 2 Zn Pb K 8 to 25 1 to 5 Si Cu 2 0.1 to 0.4 Al Cd 0.1 to 0.05 to 0.2 Cl 3 to 10 Cr 0.03 to 0, 0.1 to 1.5 to 0.5 to 3 to 10 Ca High levels zinc and lead from these dusts, associated with the very large quantities of these dusts - 15 to 20 kg / T of steel or several million tons / year in the world - encourage to think about their valuation. Several processes have been studied for this purpose in the last 50 years, by pyrometallurgical or hydrometallurgical acid or alkaline, without major success, except the Waelz process, implementing carbon reduction / oxidation in rotary kilns. The simplicity and the production of a half-product - zinc and lead oxides (60% Zn and 10% Pb) - made the success of the Waelz process.

The patent application WO 99/53108 reviews various processes for treating steelworks dust, in particular electrical steelworks. Other processes for the treatment or recovery of dust from electrical steelworks are described in patent applications EP 441 051, EP 453 151, EP 1 439 237, FR 2 757 540, WO 98/01590, WO 01/06299 and WO 2005/059038.

However, these various processes lead to the recovery of metal mixtures containing significant amounts of chlorine and fluorine, because of the abundance of these two elements in the dust of electric steel mills. Chlorine and fluorine are poisons for acid leaching plants in zinc plants. Indeed, in acidic medium, the chlorides and the fluorides are very corrosive and besides the problems of chemical engineering which they pose, the release of chlorine and fluorine at the anode of the electrolyses is not acceptable. Another problem lies in the proportion of zinc combined with total iron / zinc in electric steel mill dust, a proportion that can vary from 25 to 75% depending on the steelworks. In fact, zinc combined with iron in the form of ferrite (ZnFe 2 O 4) is more difficult to mobilize than zinc in the form of zincite (ZnO); the yield of zinc obtained by conventional sulfuric leaching (H2SO4 25%) varies according to the dust of electric steelworks from 25 to 85%. Zinc ferrites resistant to cold attack, it is necessary to carry out hot sulfur leaching (90/95 ° C) to obtain an acceptable yield of zinc (> 92 the immediate consequence is the solubilization of iron: ZnFe2O4 + H2SO4 ZnSO4 + Fe2 (SO4) 3 + H2O The separation Zn / Fe then requires the precipitation of iron by neutralization at pH> 4, in the form of goethite (hydroxide) very difficult to filter or jarosite easier to filter but bulky and Thus, a large amount of ferrite in the dust causes poor performance or a problem of residues to be eliminated Summary of the Invention One of the objectives of the present invention is therefore to significantly reduce the chlorine and fluorine content. electric mill dust after washing with water by promoting the formation of water-soluble sodium chloride and fluoride Another object of the present invention is to minimize the zinc ferrite presence in the dust to minimize solubilization of the iron in the acid leach.

Another object of the present invention is to enhance the dust of electric steel mills by enriching them with zinc and lead. These objectives are achieved by implementing the method according to the invention described below. The invention therefore relates to a method of treating electric steel mill (PAE) dusts by partial recycling said dust, after agglomeration, in the electric furnace for the production of steel. The method according to the invention consists in particular in recovering part of the PAE at the outlet of the bag filter intended to treat the steelworks fumes, to agglomerate the PAE recovered using a source of sodium oxide ( Na2O) and a carbon source, and then reintroduce the compacted PAE into the aforementioned electric furnace. Non-recycled PAE at the end of the new heat treatment can advantageously be washed with water. DESCRIPTION OF THE FIGURES FIG. 1 is a schematic representation of a conventional method of producing steel.

Figure 2 schematically shows the method according to the invention. Figure 3 schematically shows a particular embodiment of the method according to the invention. DESCRIPTION OF THE INVENTION FIG. 1 illustrates the process conventionally carried out in so-called "second-molar" electrical steelworks, in which recovery scrap is melted in the presence of additives (lime and fluorine) in an electric furnace. (1), at temperatures generally between 1200 ° C and 1600 ° C. In this process, the slag is removed and the steel is recovered. The gases produced during the production of steel contain particles and / or dust These gases must be purified before being discharged into the atmosphere. For this purpose, a bag filter (2) is used which retains the dust contained in the gases. The valorization of dust from electrical steelworks is low insofar as it requires a pyrometallurgical and / or hydrometallurgical treatment whose cost is not negligible. Thus, a very large proportion of electric steel mill dust is stored in a landfill suitable for the storage of industrial waste (class 1) or reprocessed by the Waelz process. The present invention aims to make more efficient and less expensive the dust of electric steel mills while reducing the amount of dust produced and therefore limiting the possible impact on the environment of a storage of such waste. Thus, the invention relates to a process for recovering the dust from electric steelworks derived from second-stage electric steel furnace furnaces, in which part of the dust collected in a baghouse filter (12) is agglomerated in the presence of a source. of sodium oxide (Na2O) and a source of carbon to form balls, the balls thus obtained being then introduced into the electric furnace (13). According to the process of the invention, about 30% to about 75% (by weight), preferably about 40% to about 60%, of the amount of dust collected in the baghouse (12) is agglomerated in the presence of a source of sodium oxide (Na2O) and a carbon source. The remainder of the dust can be stored after having been advantageously washed to remove soluble chlorides and fluorides in the water. The source of sodium oxide is selected from NaOH or Na2CO3. Sodium oxide, very alkaline, only exists at high temperature, in the absence of water or carbon dioxide; at high temperature, sodium hydroxide and sodium carbonate decompose to sodium oxide: 2NaOH - * Na2O + H2 - Na2CO3 ---> Na2O + CO From a practical point of view, the preferred source of sodium oxide is sodium carbonate because the evolution of hydrogen at high temperature is undesirable. Advantageously, about 1% by weight to about 5% by weight, preferably about 3% to about 4% by weight of sodium oxide source is used, based on the weight of dust to be agglomerated. As a guide, when sodium carbonate is used, it is generally in the form of a solution, for example a concentrated solution at about 250 g / l. The carbon source can be chosen from pulverized coal, heavy fuel oil, pitch, sawdust or a mixture of these materials. The pulverized coal is a preferred carbon source for carrying out the process of the invention. Advantageously, about 10% by weight is used at about 20% by weight of carbon source, based on the weight of dust to be agglomerated. The agglomeration of the dust in the presence of the source of sodium oxide and the carbon source can be carried out according to techniques well known to those skilled in the art. For example, dusts from the baghouse, the source of sodium oxide and the carbon source are mixed with a twin-screw kneader screw, and the mixture is ball-shaped using a bricklayer. The mixer is fed by means of a feeding screw and the mixture is agglomerated under a pressure of about 150 to 200 bar.

The balls thus obtained are then introduced into the electric furnace with scrap, preferably at the bottom of the crucible to limit the dust flights at the start of heating, at a rate of about 1% by weight to about 2% by weight of balls, compared to the weight of scrap.

The dust generated by melting the scrap is captured after cooling, conventionally, in the baghouse filter (12); the amount of dust collected is obviously larger (about 30 to 40% more) but this increased amount is acceptable without modification of existing filtration facilities. Part of the dust thus collected (ie about 30% to about 75% (by weight), preferably about 40% to about 60%, of the amount of dust collected in the baghouse) is recycled as previously described, other part is conditioned to be stored. According to a variant of the process of the invention, represented in FIG. 3, the part of the dust which is not recycled (that is to say the dust intended for storage), ie approximately 25% to approximately 70% ( by weight) of the amount of dust collected in the baghouse is treated in a water washing unit (14). The washing with water may comprise the following steps: a) a mixing of the dusts in water, at a solid concentration of between about 250 g / l and about 1000 g / l, for a period generally of between about 15 minutes and about 1 h; b) filtration under pressure (typically about 15 bar), for example on a filter press, preferably equipped with membrane trays, the "product" resulting from mixing; and c) a final washing with water of the residue (dust ) obtained, by counter-current percolation, from about 0.2 to about 0.6 m3 of water per tonne of dust; d) drying of the dusts e) possibly pelletizing without additive of the finished product, to facilitate the transport and the final handling.

According to a particular embodiment, the pH can be adjusted in line to a value of about 8-9 in step c); the removal of traces of Pb and Cd in solution is ensured by the addition of Na2S in order to precipitate PbS and CdS respectively it is then necessary to filter the wash water, for example on a filter cartridge or sand filter. The various equipment and techniques necessary for the implementation of the washing described above, fall within the general knowledge of the skilled person. The process according to the invention makes it possible to limit the formation of insoluble chlorides and fluorides (PbCl2 and CaF2 for the most part) and to reduce the amount of iron present and / or to avoid the formation of ferrite by promoting the formation of hematite . The above-mentioned chlorides and fluorides result from neutralization in the fumes collected above the furnace, hydrochloric and hydrofluoric acids by Na2O, a very alkaline and highly reactive oxide, which facilitates the formation of NaCl and NaF at the expense of other chlorides and fluorides. Thus, the recycled electrical steel mill (EAF) dusts have, after washing, a Cl content <0.05% and a F content <0.01% (the% are expressed by weight, relative to the total weight of the PAE composition) . The process according to the invention also makes it possible to enrich the lead and zinc content of the dusts of electrical steelworks.

The invention will be illustrated with the aid of the example below, given purely by way of illustration. Example A semi-industrial test was performed on a sample of PAE, collected in a bag filter at the outlet of the electric oven. This sample has the following composition: Element% by weight Element% by weight Zn 25 Mg / 5 Pb 5 11Na Fe 25 11 K Cu 0.3 Si Cd 0.05 Al 2 Cr 0.3 (I CI Mn 3 11F 0, 1 Ca 3 Balls are prepared by mixing in a double screw PAE, sodium carbonate in concentrated solution at 250 g / I (80 to 400 l / h, fed by a pump) and pulverized coal (100 to 200 kg This mixture feeds a briqueteuse, by means of a feeding screw, and agglomerates under a pressure of about 150 to 200 bar to form balls.These balls are then put in an electric oven with scrap. (at a rate of about 1.5% by weight of balls, relative to the weight of scrap.) The heating of the balls leads to the following reactions: Reduction of oxides in the scrap and the melt: Fe2O3 + 3C -4 2Fe + 3CO ZnO + C -4 Zn + CO ZnFe2O4 + 4C -> Zn + 2Fe + 4CO PbO + C -4 Pb + CO

Oxidation of the vaporized metals in the fumes above the melt: 2Fe + 3/202 -4 Fe2O3 Zn + 02 - * ZnO Pb + 1/2 02 -4 PbO Zn Fe2O3 ZnFe2O4 An analysis of the dust generated by the melting of the scrap and collected, after cooling, in a baghouse, reveals the following composition: Element% by weight Element% by weight Zn 37 Mg Pb 7 Na Fe 13 K 1.5 Cu 0.5 Si 8 Cd 0.06 Al Cr 0 , 5 Cl 7.5 Mn 4 F 0.2 Ca 4 The mass balance of the process shows a significant reduction of the mass of PAE from about 20 kg / t of steel (process without dust recycling) to about 14 kg / t of steel (process according to the invention). Moreover, a cold sulfuric leaching of these PAE leads to a solubilization of 92% to 94% of the zinc, this result shows that a fraction of the iron is 0 in hematite and non-ferrite form and, therefore, that the recombination Fe2O3 / ZnO is partial. The explanation for this partial recombination lies in the formation of sodium zincate, with the excess of sodium oxide available according to the following reaction scheme: Na 2 O + ZnO → Na 2 ZnO 2 in competition with the formation of zinc ferrite: ZnO + Fe2O3 -> ZnFe2O4 The addition of sodium carbonate during pelletization thus promotes the formation of zincate and limits the formation of zinc ferrite. Thus, in addition to the neutralization of chlorine and fluorine, pelletization with a source of Na2O, in this case Na2CO3, makes it possible to reduce the proportion of insoluble zinc and provides reducing capacity by decomposing the carbonate to carbon monoxide. It can also be noted a reduction in the amount of dioxins (cyclic organochlorine compounds) present; in fact, the consumption of chlorine by Na2O limits the available chlorine for the formation of dioxins. The "recycled" PAE are then washed using the following steps: - mixing PAE in water, at a solid concentration of about 1000 g / l, for about 30 min - pressure filtration of about 15 bar on a filter press equipped with membrane trays, the "product" resulting from mixing - final washing with water by percolation against the current, at the rate of about 0.5 m3 of water / tonne of PAE; - drying of PAE. After washing, the mass yield is 88% and the composition of the PAE is as follows: Element% by weight Element% by weight Zn 42 Mg 0.5 Pb 8 Na 0.1 Fe 14.5 K 0.1 Cu 0, 6 If 9 Cd 0.06 Al 3.5 Cr 0.6 Cl 0.04 4.5 F 0.01 Ca 4.5 Moisture 15 This type of product may be valued to the extent that it is likely to be used in zinc production plants by sulfur hydrometallurgy. In addition, the washing water (pH = 8.5) has the following characteristics: Element mg / l Element mgji Zn 0.4 Mg <4.0 Pb <0.4 Na 26500 Fe 0.3 K 9300 Cu 0, 3 If <12 Cd <0.4 Al 0.6 Cr <0.1 Cl 49500 Mn 0.2 F 1270 Ca 220 These wash waters are dischargeable at sea after a pH adjustment (using HCl or CO2), possibly after safety sulphurization and finishing filtration.

Claims (2)

REVENDICATIONS1. Process for recovering dust from electric steelworks from second-stage electric steel furnaces, in which part of the dust collected in a baghouse (12) is agglomerated in the presence of a source of sodium oxide (Na2O ) and a carbon source to form balls, the balls thus obtained being then heated in an electric furnace (11) with scrap and collected in the bag filter (12). The process according to claim 1, wherein the amount of dust collected in the baghouse (12) used to form the balls is from about 30% to about 75% (by weight), preferably from about 40% to about 40% by weight. % and about 60% (by weight), based on the total amount of dust collected. The process of claim 1 or 2, wherein the source of sodium oxide is selected from NaOH or Na2CO3, and is used in an amount of from about 1% by weight to about 5% by weight, preferably about 3% by weight. about 4%, based on the weight of dust to be agglomerated. 4. Method according to one of claims 1 to 3, wherein the carbon source is selected from pulverized coal, heavy fuel oil, pitch, sawdust or a mixture of these materials, and is used in a quantity from about 10% by weight to about 20%, based on the weight of dust to be agglomerated. Process according to one of Claims 1 to 4, in which the weight ratio between the balls and scrap charged in the oven (11) is between approximately 1: 100 and approximately 2: 100. 6. Process according to one of the Claims 1 to 5, wherein a portion of the dust collected in the baghouse (12) after heating the balls with the scrap is washed with water. The method of claim 6, wherein the washing with water comprises the following steps: a) mixing the dusts in water at a solid concentration of between about 250 g / l and about 1000 g / l; b) pressure filtration of the product resulting from the mixing; c) a final washing with water of the residue obtained by percolation against the current, at a rate of about 0.2 to about 0.6 m3 of water / tonne of dust; ) a drying of the dust.