EP2614165A1

EP2614165A1 - Method for recycling dust from electric steel plants

Info

Publication number: EP2614165A1
Application number: EP11773034.1A
Authority: EP
Inventors: Georges Schmickrath
Original assignee: Associates Researchers and Engineers
Current assignee: Associates Researchers and Engineers
Priority date: 2010-09-06
Filing date: 2011-09-06
Publication date: 2013-07-17
Also published as: FR2964392A1; JP2013543052A; KR20130105844A; WO2012032256A1; FR2964392B1

Abstract

The invention relates to a method for recycling dust from electric steel plants, originating from the furnaces of secondary-smelting electric steel plants, in which a portion of the dust collected after smelting in the electric furnace is agglomerated in the presence of a sodium oxide source and a carbon source so as to form pellets, and the resulting pellets are subsequently fed into the electric furnace with the scrap.

Description

Process for recovering dust from electrical steelworks

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

Electrical steel mill dust is recovered by bag filters used to capture the smoke ash from 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 composition of steel mill dust can be illustrated as follows:

The high levels of zinc and lead in these dusts, combined with the very large quantities of these dusts - 15 to 20 kg / T of steel, which is several million tons / year worldwide - encourage us to think about their recovery.

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 a 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, and in US5942198.

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 · O) is more difficult to mobilize than zinc in the form of zincite (ZnO); the yield of zinc obtained by conventional sulfuric leaching (H ₂ S0 ₄ 25%) varies according to the dust of electric steelworks from 25 to 85%. Zinc ferrites resistant to cold attack, it is necessary to practice hot sulfuric leaching (90/95 ° C) to obtain acceptable zinc yield (>92%); the immediate consequence is the solubilization of iron;

ZnFe ₂ O ₄ + H, SO ₄ → ZnSO ₄ + Fe ₂ (SO ₄ ) ₃ + H ₂ O

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 bulky. Thus, a large amount of ferrite in the dust causes poor performance or a problem of residues to eliminate.

Summary of the invention

One of the objectives of the present invention is therefore to significantly reduce the chlorine and fluorine content of electric steel 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 presence of zinc ferrite in the dust to minimize the solubilization of iron during acid leaching.

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 PAE recovered with a source of sodium oxide 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 figures

Figure 1 schematically shows 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-melting" electric steelworks, in which recovery scrap is melted in the presence of additives (lime and fluorine) in an electric furnace (1), temperatures generally between 1200 ° C and 1600 ° C. In this process, the slag is removed and the steel is recovered. 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 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 and a source of carbon. The remainder of the dust can be stored after having been advantageously washed to remove soluble chlorides and fluorides in the water. The term "agglomerated in the presence of a source of sodium oxide and a source of carbon" means that the PAEs are agglomerated only with the source of sodium oxide and the carbon source.

The source of sodium oxide is chosen from NaOH or Na ₂ CO ₃ . Sodium oxide, very alkaline, only exists at high temperature, in the absence of water or carbon dioxide; at high temperature, soda and sodium carbonate decompose into sodium oxide:

2NaOH → Na, O + H ₂ T

Na ₂ CO ₃ → Na ₂ O + CO ₂

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. By way of indication only, when sodium carbonate is used, it is generally in the form of a solution, for example a concentrated solution at approximately 250 g / l.

According to one embodiment of the invention, the source of sodium oxide is replaced by potassium carbonate, that is to say that the portion of the dust collected in the baghouse (12) is agglomerated in the presence of of potassium carbonate and the carbon source to form balls. Potassium carbonate is generally used in amounts comparable to those indicated above for the source of sodium oxide.

The carbon source may be selected from pulverized coal, heavy fuel oil, pitch, sawdust or a mixture of these materials; pulverized coal is a preferred source of carbon for the implementation of 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 (13) with scrap metal, preferably at the bottom of the crucible to limit the outflows of dust at the beginning of the heating, at a rate of about 1% by weight to about 2% by weight. of balls, in relation 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). Washing with water may include the following steps; a) kneading the dusts in water, at a solid concentration of between about 250 g / l and about 1000 g / l, for a period generally between about 15 min and about 1 h;

b) filtration under pressure (typically of about 15 bar), for example on a filter press, preferably equipped with membrane trays, the "product" resulting from mixing;

c) a final wash with water of the residue (dust) obtained, by percolation against the current, at a rate of about 0.2 to about 0.6 m ³ of water / tonne of dust;

d) drying of the dust;

e) optionally pelletizing without additive of the finished product, to facilitate the transport and the final handling.

This variant is particularly advantageous for reducing the chlorine and fluorine content of the PAEs.

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 Na ₂ S in order to precipitate PbS and CdS respectively; it is then necessary to filter the washing water, for example on a filter cartridge or sand filter.

The various equipment and techniques required for the implementation of the washing described above, are general knowledge of the skilled person. The process according to the invention makes it possible to limit the formation of insoluble chlorides and fluorides (PbCb and CaF, 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 chlorides and fluorides mentioned above result from neutralization in the fumes collected above the furnace, hydrochloric and hydrofluoric acid by Na. very alkaline and very reactive oxide, or by K ₂ 0, which facilitates the formation of NaO and NaF, or KCl and KF, to the detriment of other chlorides and fluorides. Thus, recycled electric steel mill (EAF) dusts present, after washing, a content of Cl <0.05% and a content of F <0.01% (the% is 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 trial was carried out on a sample of PAE, collected in a bag filter at the outlet of the electric oven. This sample has the following composition:

Balls are prepared by mixing in a double screw PAE, sodium carbonate in concentrated solution at 250 g / l (80 to 400 l / h, fed by a pump) and pulverized coal (100 to 200 kg / h) . 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 charged 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 scrap and the melt:

ZnO + C → Zn + CO

ZnFe ₂ 0 ₄ + 4C → Zn + 2Fe + 4CO

PbO + C → Pb + CO

Oxidation of the vaporized metals in the fumes above the melt 2Fe + 3/20 ₂ -> Fe ₂ 0 ₃

Zn + ½ 0 ₂ → ZnO

Pb + ½ 0 ₂ → PbO

ZnO + Fe ₂ 0 ₃ → ZnFe ₂ 0 ₄

An analysis of the dust generated by the melting of scrap and captured, after cooling, in a bag filter reveals the following composition;

The mass balance of the process shows a significant reduction in 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).

In addition, a cold sulfuric leaching of these PAE leads to a solubilization of 92% to 94% of zinc; this result shows that a fraction of the iron is in hematite and non-ferrite form and, therefore, that Fe recombination _; 0 _; / 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 → a. ZnO _?

in competition with zinc ferrite formation:

ZnO + Fe ₂ 0 ₃ → ZnFe ₂ 0 ₄

The addition of sodium carbonate during pelletizing thus promotes the formation of zincate and limits the formation of zinc ferrite.

Thus, besides the neutralization of chlorine and fluorine, pelletization with a source of Na ₂ O, in this case Na ₂ CO ₃ , makes it possible to reduce the proportion of insoluble zinc and provides reducing capacity by decomposition of the carbonate. carbon monoxide. It can also be noted a reduction in the amount of dioxins (cyclic organochlorine compounds) present; indeed, the consumption of chlorine by Na ₂ 0 limits the available chlorine for the formation of dioxins.

"Recycled" EAPs are then washed using the following steps:

mixing PAE in water at a solid concentration of about 1000 g / l for about 30 minutes;

filtration under pressure of about 15 bar, on press filter 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 m ³ 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 Si 9

Cd 0.06 Al 3.5

Cr 0.6 Cl 0.04

Mn 4.5 F 0.01

Ca 4.5 humidity 15

This type of product can be valorized insofar as it is likely to be used in zinc production plants by sulfur hydrometallurgy.

In addition, wash water (pH = 8.5) has the following characteristics

These wash waters are discharged into the sea after pH adjustment (using HCI or CC½), possibly after safety sulphurization and finishing filtration.

Claims

1. Process for recovering dust from electric steelworks from secondary 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 (Na ₂ 0) and a carbon source to form balls, the balls thus obtained being then heated in an electric furnace (11) with scrap and then 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 according to claim 1 or 2, wherein the source of sodium oxide is selected from NaOH or Na ₂ CO ₃ and is used in an amount of from about 1% by weight to about 5% by weight, preferably about 3% to about 4%, based on the weight of dust to be agglomerated.

4. The method of claim 1 or 2, wherein the source of sodium oxide (Na ₂ 0) is replaced by potassium carbonate.

5. Method according to one of claims 1 to 4, 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.

6. Method according to one of claims 1 to 5, wherein the weight ratio between the balls and the scrap baked in the oven (11) is between about 1: 100 and about 2; 100.

7. Method according to one of claims 1 to 6, wherein a portion of the dust collected in the baghouse (12) after heating the balls with scrap is washed with water,

8. The method of claim 7, wherein the dusts of electric steel mills have, after washing, a content of Cl <0.05% and a content of F <0.01% (% expressed by weight, relative to the total weight the composition of electric steel dust).

The method of claim 7 or 8, wherein the water washing 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 wash with water of the residue obtained by percolation against the current, at a rate of about 0.2 to about 0.6 m ³ of water / ton of dust;

d) drying of the dust.

A process for reducing the chlorine and fluorine content of electric steel mill dust from secondary electric steel furnaces, wherein (i) a portion of the dust collected in a baghouse (12) is agglomerated into presence of a source of sodium oxide (Na ₂ O) and a source of carbon to form balls; (ii) the balls thus obtained are then heated in an electric oven (11) with scrap and then collected in the baghouse (12); and (iii) a portion of the dust thus collected is washed with water.

11. The method of claim 10, wherein the amount of dust, collected in the baghouse (12), used to form the balls is included between about 30% and about 75% (by weight), preferably between about 40% and about 60% (by weight), based on the total amount of dust collected.

The process according to claim 10 or 11, wherein the source of sodium oxide is selected from NaOH or Na ₂ CO ₃ , and is used in an amount of about

1% by weight to about 5% by weight, preferably about 3% to about 4%, based on the weight of dust to be agglomerated.

13. The method of claim 10 or 11, wherein the source of sodium oxide (Na ₂ 0) is replaced by potassium carbonate.

The method according to one of claims 10 to 13, 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.

15. Method according to one of claims 10 to 14, wherein the weight ratio between the balls and the scrap baked in the furnace (11) is between about 1: 100 and about 2: 100.

16. The method according to one of claims 10 to 15, wherein the washing with water comprises the following steps:

b) pressure filtration of the product resulting from the mixing;

d) drying of the dust.

17. Method according to one of claims 10 to 16, wherein the dusts of electric steel mills have, after washing, a content of Cl <0.05% and a content of F <0.01% (% expressed by weight , relative to the total weight of the composition of the dust of electric steelworks).