WO2014044993A1

WO2014044993A1 - Method for the hydrometallurgical processing of steel plant dust resulting from scrap metal smelting

Info

Publication number: WO2014044993A1
Application number: PCT/FR2013/052246
Authority: WO
Inventors: Georges Schmickrath
Original assignee: Mine & Ore
Priority date: 2012-09-24
Filing date: 2013-09-24
Publication date: 2014-03-27
Also published as: FR2995911A1

Abstract

The invention relates to a method comprising the steps of: a) collecting dust by filtering (12) the gases leaving a scrap smelting furnace; b) preparing (14) a mixture containing a first fraction of dust collected in step (a), a carbon source and a sodium oxide source; c) preparing (16) briquettes by means of the pressure compacting of the mixture obtained in step (b); d) feeding the briquettes into the scrap smelting furnace and pyrometallurgically processing (10) the briquettes with the scrap; and e) performing hydrometallurgical processing (20-36) for dissolution of the chlorides and fluorides of a second fraction of dust collected in step (a) but not used for the mixture in step (b), with the addition of a sodium source reagent to said second dust fraction for hydrometallurgical processing (20-36) for the dissolution of the chlorides and fluorides.

Description

Process for the hydrometallurgical treatment of steelworks dust resulting from the melting of scrap metal

The invention relates to the recovery of steel mill dust, more specifically dust resulting from the melting of scrap recovery in electric furnaces.

In these installations, the smoke ash from secondary melting furnaces is recovered by appropriate filtration systems to give electric steel mill or "PAE" dusts.

The composition of these EAPs reflects that of the recovered scrap with which the furnace was fed, including automobile carcasses: mild and stainless steels, chromed, phosphated, cadmium and / or electrozinc, alloyed, mixed with copper and their connectivity, lead batteries, plastics (PVC, PE, PP, PTFE, PVDF, ...), paints, etc. Or, when the dust comes from building waste: mild steels, galvanized, phosphated, mixed with mineral waste such as concrete or silico-aluminous, etc.

These EAPs contain varying amounts of elements such as zinc, iron, lead, copper, manganese, chromium, cadmium, chlorine and fluorine, the variability depending on the quality of the baked scrap. A typical composition of PAE is given in the following Table I:

Table I

Element% by weight Element% by weight

Zn 18 to 30 Mg 0.1 to 2

Pb 2 to 8 Na 1 to 5

Fe 8 to 40 K 1 to 5

Cu 0.1 to 0.4 Si 0.2 to 1

Cd 0.05 to 0.2 Al 0.1 to 0.5

Cr 0.03 to 0.2 Cl 3 to 10

Mn 0.5 to 3 F 0.1 to 1, 5

Ca 1 to 10 These dusts are produced in very large quantities, of the order of 15 to 20 kg of dust per tonne of steel. They are further classified as hazardous waste by the various existing legislations (Basel Convention hazardous waste list A, code K061 according to the United States Environmental Protection Agency).

This large amount of PAE generated implies considerable needs in terms of availability of temporary or permanent storage space, transport and treatment, and also causes significant environmental and health nuisances: emission of ef- greenhouse gases, risks related to the transport of hazardous waste, etc.

It is desirable to upgrade these EAPs, and economically viable given their high content of iron, zinc and lead.

Numerous processes for recovering these metals, by pyrometallurgical or hydrometallurgical acid or alkaline route, have been proposed, which are described for example in WO 99/53108 A1, EP 0 453 151 A1, EP 1 439 237 A1, FR 2 757. 540 A1, WO 98/01590 A1 or WO 2005/059038 A1.

Another particular technique of upgrading PAE is described in WO 2012/032256 A1 (Associates Researchers and Engineers), the same inventor as the previous application.

The technique proposed by this document consists in mixing a fraction of the PAEs captured by the filter not only with a source of carbon (pulverulent coal, heavy fuel oil, pitch, sawdust, etc.), but also with an oxide source. sodium Na ₂ O (sodium carbonate Na ₂ CO 3 or sodium hydroxide NaOH). The mixture is agglomerated under pressure to form balls which are then charged in the electric oven with the scrap at a rate of about 1 to 2% by weight of pellets with respect to scrap.

The source of sodium oxide makes it possible in particular to reduce the proportion of insoluble zinc: in fact, the zinc contained in the PAE can be in oxidized form, zincite ZnO, but also combined with iron in the form of ferrite ZnFe ₂ O ₄ , can represent from 25 to 75% of the total zinc in the PAE according to the steelworks. However, in ferrites, zinc is more difficult to mobilize, because zinc ferrites resist cold attack by sulfuric acid, and it was previously necessary to carry out a leachate. hot sulfuric acid to obtain an acceptable zinc yield. This had the effect of solubilizing the iron, which was then separated from the zinc, leading to the formation of new compounds (goethite, jarosite) very difficult to filter or bulky and bulky. In the presence of a large amount of ferrites, the technique described in WO 2012/032256 A1, through the addition of a source of sodium oxide, has made it possible to improve the yield and to limit the quantity of residues. Ultimate to eliminate.

A first object of the invention is, in general, to improve the efficiency and the efficiency of the process described in WO 2012/032256 A1, in particular by:

reduction of the tonnages and volumes of the PAEs generated: compared with the previous process which leads to a volume reduction limited to 40% on average, it will be seen that the process of the invention makes it possible to achieve a reduction up to 50 to 60% of waste quantities after treatment of EAPs;

- Immediate recovery of 80 to 90% of the iron contained in the PAE, in the form of liquid iron and recovered directly with the usual steel casting, with an increase in the recovery efficiency of the iron compared to the process previous, and in the range of 0.5% to 0.7% of steel production;

- reducing the presence of insoluble zinc ferrites in the residual dust, so as to increase the percentage of zinc available in the treated PAE and minimize the presence of solubilizable iron during acid leaching;

lowering the bursting temperature of these ferrites, thus avoiding finding them unreduced in the steel or in the slag produced by the furnace, with the risk of creating faults;

- improving the efficiency of recycling of PAE by delaying the combustion of the carbon source introduced into the mixture, so as to use carbon primarily as a reducing agent and not exhaust it as a fuel before the reduction temperatures iron oxides and zinc oxides could not be reached; - Reduction of residual levels of chlorine and fluoride in retreated PAE, by further promoting the formation of chlorides and fluorides soluble; and

- recovery of residual PAE after treatment, by enriching them strongly with zinc and lead, typically to a level as high as that of a high quality natural ore.

Another object of the invention is the improvement of the cohesion of the agglomerates intended for recycling (balls or briquettes), and this without resorting to an additional binding agent such as molasses or polyvinyl alcohol.

Indeed, it is necessary to release the least amount of dust possible during the steps of handling, storage and introduction of briquettes in the oven, to reduce the rate of breakage and premature bursting, to reduce dust surges in case of bursting due to the temperature, and to optimize the reaction time of the briquettes in the electric oven. Another disadvantage of the process described by the aforementioned WO 2012/032256 A1 is that, from the point of view of the thermodynamics and the kinetics of the reactions involved, with constant energy flow in the furnace, the reduction of oxides iron requires more time than simple melting of scrap, because of intermediate reactions and elements present in different phases, which must react together.

This has the effect of increasing the duration of the melting cycle of scrap between two flows ("tap-to-tap"), which greatly affects the productivity of the steel mill. On the other hand, the iron oxides not reduced due to a residence time in the oven too short are likely to end up in the liquid steel or slag, thus increasing the impurities in the steel when casting, with consequent reduction in the quality of the finished or semi-finished products of the steel mill and a reduction in the recovery efficiency of the iron.

Another object of the invention is to remedy this drawback, and it will be seen in particular that the implementation of the teachings of the invention allows the steel mill to maintain a tap-to-tap identical to that of the merger a usual load of scrap, despite the addition in the furnace briquettes recycled PAE, and this while improving the recovery of iron recovery. According to another of its aspects, the invention relates to optimizing the performance of the hydrometallurgical treatment of the fraction of PAE that has not been agglomerated into briquettes for recycling in the oven.

Indeed, the treatment described above (pyrometallurgical treatment) concerns only a fraction of PAE. This remaining fraction, which is of the order of 15 to 70%, typically 40 to 60% of the total PAE captured by the filter, will be called thereafter "second fraction", the "first fraction" being that which after mixing and Briquetting is recycled in the furnace for direct pyrometallurgical treatment.

This second fraction is insusceptible of an immediate valuation, in particular for the recovery of zinc and lead, because of the high residual contents in chlorine and fluorine, and this even with the method described in WO 2012/032256 A1 cited above, which led to a chlorine content still too high (of the order of 0.5% or 5000 ppm) after treatment in a washing unit with water.

Indeed, because of their origin, the recovered scrap contains significant amounts of chlorine and fluorine, as can be seen from the composition typical of Table I above. Chlorine and fluoride, which are highly corrosive, are dangerous to workers' health and seriously damage the acid leaching facilities that electrolytically produce zinc from raw or previously treated PAE. In addition, the release of chlorine and fluorine at the anode of the electrolysis units is not acceptable given the risks involved.

These contents therefore do not allow a direct treatment of recovery of zinc, so that after washing it is necessary to mix the PAEs with natural ores, for example ZnS, in a proportion of the order of 15% of waste for 85 % ore, then toast the mixture to remove chlorine and fluorine.

With the prior process, there was therefore a maximum limit of integration of PAEs to 15% of the total amounts of materials treated with zinc smelters.

The subject of the invention is, according to a second of its aspects, a hydrometallurgical treatment process which eliminates this limitation of the capacities of PAE integration in zinc smelter production processes.

Thus, another object of the present invention is to improve the hydrometallurgical process described in WO 2012/032256 A1 for treating the second fraction of the PAE so as to have, at the outlet of the treatment, leached and dried dust suitable for a direct zinc recovery treatment, thanks to a very low rate of residual chlorine and fluorine, typically 300 to 500 ppm of chlorine by the process of the invention (instead of 5000 ppm by the prior method).

This improvement results in part, as will be seen below, from the improvement of the process at the stage of the pyrometallurgical treatment of the first fraction, which has the effect of reducing the formation of insoluble chlorides and fluorides (essentially PbC ^ and CaC ^) in the PAEs extracted from the oven fumes.

However, it appears desirable to further reduce the level of chlorine and residual fluorine in the hydrometallurgical treatment of the second fraction, operated downstream, by improving this phase of the process so as to have a final product that can be directly processed for recovery zinc or lead, without any other intermediate treatment such as mixing with natural ore and calcining.

It is in particular:

to increase the rate of removal of fluorine from 25 to 30% of dissolution yield (with the prior method) to 50% (with the process of the invention); and

- to lower the minimum residual chlorine level from 5000 ppm to 300 to 500 ppm,

by displacing the thermodynamic equilibrium of solubilization of chlorides and fluorides in the direction of the formation of soluble salts of chlorine and fluorine which will be easily eliminated by lixiviation.

To achieve all these objects, the invention proposes a method for treating PAE comprising, as disclosed by the aforementioned WO 2012/032256 A1, steps of:

a) collecting dust by filtration of the gases produced at the outlet of a scrap melting furnace; b) preparing a mixture comprising: a first fraction of the dusts collected in step a), a source of carbon, and a source of sodium oxide;

c) preparing briquettes by agglomeration under pressure of the mixture obtained in step b);

d) briquetting briquettes in the melting furnace receiving the scrap, and pyrometallurgical treatment briquettes with scrap; and

e) hydrometallurgical treatment for the dissolution of chlorides and fluorides of a second fraction of the dust collected in step a), not used for the mixture of step b).

Typically, according to a first aspect of the invention, step b) comprises adding a combined source reagent of sodium oxide and silicon oxide, which reagent is a unique complex source of sodium oxide and silicon selected from: sodium metasilicate Na ₂ SiO ₃ , sodium orthosilicate Na ₄ SiO ₄ , sodium pyrosilicate Na ₆ Si ₂ O ₇ and mixtures of the foregoing.

According to various advantageous subsidiary implementations of this first aspect of the invention:

the source of sodium oxide is added in excess, the molar percentage of the combined source of sodium oxide and of silicon oxide in the mixture of step b) being such that the sum of the molar amounts of sodium and potassium contained in the first fraction of the dust and those contained in the combined source of sodium oxide and silicon oxide exceeds by 10 to 50%, preferably 20 to 30%, the sum of the molar amounts of chlorine and fluorine contained in the first fraction of the dust;

the SiO ₂ / Na ₂ O mass ratio of the complex source of sodium oxide and silicon oxide is less than 3.5;

the combined source of sodium oxide and silicon oxide comprises a source of sodium oxide and a source of discrete silicon oxide, the source of sodium oxide being chosen in particular from sodium hydroxide NaOH and sodium carbonate Na2CO3, and the source of silicon oxide being in particular silica S102; the mixture of step b) comprises 1 to 5%, preferably 3 to 4%, of the complex source of sodium oxide and silicon oxide with respect to the mass of the first dust fraction;

the pressure agglomeration of step c) is carried out without the addition of a binding agent to the mixture prepared in step b);

- The briquetting of the briquettes in step d) is performed at the bottom of the oven bath, before introduction of scrap;

the first fraction of the dust used in step b) for the preparation of the mixture is a fraction of 30 to 85%, preferably 40 to 60%, of the dust collected in step a).

Also typically, according to a second aspect of the invention, for the hydrometallurgical treatment of dissolution of the chlorides and fluorides of said second fraction of the dusts, it is provided in step e) the addition of a sodium source reagent to the second fraction of dust.

The sodium source reagent may be selected from sodium hydroxide NaOH and sodium carbonate Na2CO3.

In a different field, US 5,942,198 A discloses a steel dust treatment operation employing sodium hydroxide treatment. But it is in this document to dissolve the zinc in the dust in order to recover it and purify it. In the case of the invention, it is not a question of dissolving the zinc but of eliminating the chlorine and the fluorine present in the dust, this elimination of the chlorine and the fluorine being carried out by lixiviation, that is to say to say by elimination with a washing water. It is not a dissolution as in US 5 942 198 A, which dissolution would retain an element in the product obtained in order to recover it in purified form.

For carrying out the invention, the sodium source reagent is preferably added in excess, the molar percentage of the sodium source reagent added in step e) being such that the sum of the molar amounts of sodium and potassium contained in the second fraction of the dust and those contained in the sodium source reagent exceeds, advantageously from 10 to 50%, preferably 20 to 30%, the sum of the molar quantities of chlorine and fluorine contained in the second fraction. dust. The hydrometallurgical treatment of step e) can in particular comprise steps of: mixing in the water of the second fraction of the dust; leaching (s) in the basic phase of the residue obtained with the addition of the sodium source reagent; washing (s) with water the residue obtained; filtration (s) under pressure of the resulting product; washing (s) with water the residue obtained; insufflation (s) of air under pressure in the filter chambers; and drying the dust.

It may also include an optional final step of non-additive briquetting, under residual moisture, of the final product obtained in order to avoid its dispersibility and the risk that it could cause to the environment.

0

An embodiment of the invention will now be described with reference to the appended drawings in which the same references denote identical or functionally similar elements in the figures. Figure 1 is a diagram illustrating the method of treatment of PAE according to the state of the art.

FIG. 2 illustrates the various stages of pyrometallurgical and hydrometallurgical treatment of the process according to the invention.

0

FIG. 1 illustrates the various steps of a method according to the state of the art, as taught by the aforementioned WO 2012/032256 A1.

This is a conventional process used in the so-called "secondary melting" steel mills, where recovery scrap is melted in the presence of additives (lime, fluorine, etc.) in an electric furnace at low temperatures. temperatures generally between 1200 ° C and 1650 ° C (block 10), the steel being recovered after removal of the slag.

The production of steel produces gases containing particles and dust, and these gases must be purified before being discharged into the atmosphere. This purification is performed by a filter (block 12) for example a bag filter, optionally supplemented by an activated carbon filter or an electrostatic filter. Dust retained (PAE) are recovered and processed for recovery, especially for later recovery of zinc contained in these EAPs.

A first fraction of the PAE is mixed with i) a source of carbon and ii) a source of sodium oxide Na 2 O (sodium carbonate Na 2 CO 3 or sodium hydroxide NaOH) in a mixer (block 14), then agglomerated (block 16) in the form of individual elements such as balls which will be recycled in the electric furnace with the melting recovery scrap at the same time as the latter.

The proportion of balls that can be recycled in the furnace is limited, only part of the EAP is recycled. The rest of the PAE constitutes a second fraction which is washed with water in a washing unit (block 18), to give after drying of the dust which can be the subject of a recovery treatment by recovery including zinc and lead contained therein.

The disadvantages and limitations of this known process have been discussed in the introduction.

Pyrometallurgical Treatment of a First Fraction of the PAEs The improvement brought by the process of the invention to the known treatment which has just been described will now be described with reference to FIG.

The melting and filtration stages of the dust-laden gases (blocks 10 and 12) are implemented in the same manner as previously. The first fraction of the PAE, intended to be recycled into the melting furnace with the scrap, is also mixed and agglomerated (blocks 14 and 16).

The first fraction of the PAEs, which represents from 30 to 85% by weight, preferably from about 40 to 60% by weight, of the total amount of dust collected by the filter, is mixed with a carbon source and so characteristic of the invention, to a combined source of sodium oxide and silicon oxide.

The carbon source can be chosen, in itself known manner, from pulverized coal, heavy fuel oil, pitch, sawdust or a mixture of these materials. This source is preferably coal sprayed at a quantity of between approximately 10 and 30% by weight of carbon relative to the weight of the dust to be agglomerated. It should be noted that it is advantageous to have a source of carbon of small granulometry (finely divided carbon dust), so as to increase the reactivity and thus the final yield of the process.

In a characteristic manner of the invention, the combined source of sodium oxide and silicon is a complex source of sodium oxide and silicon contained in a single mineral product such as sodium metasilicate Na ₂ SiO ₃ , orthosilicate of sodium Na SiO or sodium pyrosilicate Na ₆ Si ₂ O 7, or mixtures thereof.

Specifically, the combined source of sodium and silicon oxide is preferably sodium metasilicate Na ₂ SiO ₃ which, at high temperature, is decomposed into sodium oxide and silicon oxide, according to the reaction: Na ₂ SiO ₃ → Na ₂ O + SiO ₂ .

It should be emphasized that silica having a higher affinity than sodium will, with the calcium contained in the ashes, form SiF ₄ (dilute acidic fluorosilicate), instead of CaF _2, which is insoluble and detrimental to the treatment for subsequent electrolysis. zinc metal.

The proportion of sodium metasilicate is about 1 to 5% by weight, preferably about 3 to 4% by weight, based on the weight of the dust to be agglomerated.

It is advantageous for the combined source of sodium and silicon oxide to have a low SiO ₂ / Na ₂ O mass ratio, so as to have a high content of sodium oxide. In fact, the source of sodium oxide must be introduced in excess so that the sum of the molar quantities of sodium and potassium contained in the dusts, plus those added by the combined source of sodium and silicon oxide, is greater than about 10 to 50%, preferably about 20 to 30%, to the sum of the molar amounts of chlorine and fluorine contained in the dusts.

Ideally, the SiO ₂ / Na ₂ O mass ratio is 1: 8 or 1: 10. Concretely, if one uses for example sodium metasilicate, which can be in the form of a solution with a variable concentration, this ratio is between 1, 5: 1 and 3.5: 1. An excess of silica with respect to the ideal ratio is not a problem, since after treatment it is found in the final residues in the form of silica or metal silicates, which are not troublesome.

The mixture of the first fraction of PAE, the carbon source and the complex source of sodium oxide and silicon (block 14) can be operated by means of a blender-type blender, ribbon, tank mixer rotary or double counter-rotating screws.

The mixture obtained is then agglomerated (block 16) in the form of briquettes with a briquetting machine fed by the mixture by means of a feeding screw, the agglomeration being carried out under a pressure of about 100 at 250 bars, advantageously about 200 bars.

The shape and size of the briquettes are chosen so as to optimize their processing time in the oven at the time of recycling, and to release the least amount of dust during the handling, storage and introduction into the oven, reduce the breakage rate and the premature bursting under the effect of the temperature which generates dust emissions, as well as to optimize the reaction time of the briquettes in the electric furnace (exchange surface, reduction time, time melting time, stabilization time and homogenization, etc.). In particular, briquettes of appropriate shapes and dimensions can be made having a non-angular and non-spherical shape, facilitating storage and handling without rolling, with a lateral dimension of about 3 to 6 cm, preferably about 4 cm. at 5 cm, and a thickness of about 2 to 5 cm, preferably 3 to 4 cm.

Agglomerating briquette dust is a known technique, which generally requires the use of an additional binder, such as molasses (as described in US 2010/0218420 A1) or polyvinyl alcohols (as described in EP). Above-mentioned 1,439,237 A1).

But the introduction of an additional product (the binder) in the mixture and therefore in the furnace of the steelworks often induces detrimental consequences: for example molasses causes a high moisture content in the furnace and causes deposits of soot in the gas extraction lines, which may cause explosions at this level.

Typically, the process of the invention does not require the addition of a specific binding agent to the mixture. Indeed, the choice as a complex source of sodium and silicon oxide of a sodium silicate such as sodium metasilicate provides, in addition to a (first) function of a reactive agent, a (second) function of binding agent. It is therefore necessary to introduce with the PAE in the recycled fraction, in addition to the carbon source, a single mineral product, in this case preferentially sodium metasilicate Na ₂ SiO ₃ .

The briquettes from the agglomeration stage are fed into the electric oven with the scrap in a mass proportion of up to 5% relative to the weight of the recovery scrap introduced into the furnace.

The briquettes are preferably introduced into the bottom of the crucible, advantageously in contact with the bath foot. Indeed, the introduction on the bathing foot before the introduction of scrap can maximize the residence time (about 40 to 60 minutes, depending on the operation of the steel mill) briquettes in the oven. This optimizes the energy exchanges between the briquettes and the liquid iron foot bath, briquettes recovering the inevitable heat loss of the electric oven. In addition, the introduction on the bath foot limits the dust dispersions at the beginning of heating.

The recycling of steel dust in the form of briquettes with a carbon source has been practiced for a long time, as described for example in US Pat. No. 2,417,493 A. Carbon is the reducing element which enables the reduction of metal oxides. especially iron oxides to form iron metal; carbon also provides the energy needed for briquette reactions in the furnace: reduction of metal oxides, melting of metal iron, vaporization of light elements such as zinc, etc.

It will be noted that the sodium oxide Na ₂ O produced at high temperature by the decomposition of the complex source of sodium and silicon oxide offers a (third) fluxing function in closed systems which are the agglomerated briquettes reintroduced into the furnace. electric.

This flux reduces the reaction temperature of spinels (zinc ferrite Fe ₂ O ₃ -ZnO) to about 750-850 ° C., usually from 1150-1250 ° C. In addition, the high melting power of sodium makes it possible to lower the melting temperature of the metal iron during the combustion of the metal. carbon after reduction of iron oxides in agglomerated PAE from 1538 ° C to around 1400 ° C. Finally, the fondant favors the extraction of iron from its gangue.

In addition to the three functions mentioned above:

1 °) of reactive agent,

2 °) binder and

3 °) of fondant,

the choice as complex source of sodium and silicon oxide of a sodium silicate such as sodium metasilicate provides a fourth function, which is that of:

4 °) fire retardant or flame retardant.

Indeed, the sodium metasilicate makes it possible to delay the combustion of the carbon present in the briquettes by using this carbon primarily as a reducing agent, avoiding to exhaust it as a fuel before the reduction temperatures of the iron and zinc oxides have been reduced. could be reached.

According to a particularly advantageous aspect, the process of the invention thus enables the steel mill to maintain a melting cycle of scrap between two flows (tap-to-tap cycle) identical to that of the melting of a usual load of scrap. without recycling PAE briquettes.

Indeed, with a conventional method such as that described in the aforementioned WO 2012/032256 A1, the kinetics and thermodynamics of the various reactions used for the reduction of iron oxides required more energy and more time than the simple one. melting of the ferries, in particular because of intermediate reactions during the reduction and reagents present in different phases (solid, liquid and gaseous), which slowed down the overall process.

In addition, the presence of unreduced iron oxides because of a residence time too short in the furnace led these oxides to be found in the liquid steel or in the slag, thus increasing the impurities in the steel to casting, with a corresponding reduction in the quality of the finished or semi-finished products of the steel mill and a reduction in the iron recovery efficiency.

The process of the invention, thanks to the multiple functions ensured by the choice as a complex source of sodium and silicon oxide of a silicate of sodium such as sodium metasilicate, overcomes these disadvantages by maintaining a tap-to-tap identical to that of the melting of scrap without recycling briquettes, while improving the efficiency of recovery of iron contained in the PAE, typically 80 to 90% of the iron contained in these PAEs, in the form of liquid iron and directly with the usual casting of steel.

Hydrometallurgical treatment of a second fraction of PAE The fraction of PAE, recovered after filtration of fumes from electric furnaces, which are not subject to briquette agglomeration and recycling (the "second fraction" above) are subjected to a hydrometallurgical solubilization treatment, in particular salts of chlorides and fluorides, which will now be described in detail.

This second fraction of PAE is about 15 to 70% by weight, preferably about 40 to 60% by weight, of the total amount of PAE collected.

The first step (block 20) is intensive dust mixing, to mechanically disperse the NaCl and KCl-bound agglomerates by attrition and friction in water, so as to reduce the duration of the subsequent treatment and to avoid leaching. undesired, especially metal elements, due to the time factor in a high pH solution. The solid / liquid concentration is between about 150 g / l and about 1000 g / l, and the mixing is carried out for a time of between about 15 minutes and 1 hour.

Typically, a sodium source reagent is added at this stage (block 22). This source of sodium may be sodium carbonate Na ₂ CO ₃ or sodium hydroxide NaOH.

This reagent is added in excess, in such a proportion that the sum of the molar amounts of sodium and potassium contained in the treated dusts, plus those added by this sodium source reagent, is greater by about 10 to 50%, preferably about 20 to 30%, the sum of the molar quantities of chlorine and fluorine contained in the dust. The molar excess of sodium added during this step has the effect of moving the thermodynamic equilibrium of solubilization of chlorides and fluorides in the direction of the formation of soluble chlorine and fluorine salts in the mixture, even when cold. , which can then be removed in the leachate.

These chlorides and fluorides result from the neutralization, in the fumes collected above the furnace, hydrochloric and hydrofluoric acid by the sodium oxides Na2O and silicon S1O2, very alkaline oxides and very reactive which facilitates the formation of chlorides and fluorides soluble, to the detriment of other insoluble chlorides and fluorides. The use as a reagent of a combined source of sodium and silicon oxide makes it possible in particular to introduce into the reaction chain silicon, which has a very high affinity with fluorine, a much greater affinity than that existing between sodium and silicon. fluorine.

It will be possible to obtain a fluorine removal rate in PAEs of up to 50%, and to lower the residual chlorine level to a value of about 300 to 500 ppm. These performances are far superior to those of the previous processes: thus, the process described in the aforementioned WO 2012/032256 A1 only made it possible to achieve only 25 to 30% only of the elimination rate of fluorine, with a chlorine content. residual in the final product of at least 5000 ppm.

The product resulting from the intensive mixing (attrition and friction phenomena) and the addition of the sodium source reagent is then subjected to filtration under pressure (block 22), typically at about 15 to 20 bars, for example on a Filter press preferably equipped with membrane trays.

This filtration is followed by one or more steps of final washing with water of the residue obtained (block 24), by percolation against the current at a rate of about 0.2 to 1 m ³ of water per ton of dust .

It is advantageously provided for one or more air blowing steps (block 26) under pressure of 7 to 15 bar in the chambers of the filter press, to reduce the quantities of interstitial saline waters loaded with chlorides and fluorides, in order to reduce residual levels of chlorine and fluorine and improve the quality of the finished product. The washing with water is then renewed (block 28) until the desired quality is obtained in terms of elimination of chlorine and fluorine.

The blowing of air under pressure is also renewed (block 30) until the desired quality is obtained, also in terms of elimination of chlorine and fluorine in PAE.

If necessary, the dust is then dried (block 32), unless air blowing has been sufficient.

If necessary the dust is finally briquetted (block 34) without additive but under a residual moisture of 10 to 15% of the finished product. This brickwork will facilitate the transport and handling of the final product, thus avoiding the potential risks of pollution by product flight.

EXAMPLE We will now describe an example of implementation of the invention, carried out on a semi-industrial scale on a sample of PAE collected in a bag filter at the outlet of an electric oven. This sample has a composition according to Table II below:

Table II

Briquettes are prepared by mixing PAE, sodium metasilicate in concentrated solution at 150 g / l (80 to 400 l / h, pump feed) and pulverized coal (100 to 200 kg / cm 2) in a rotary drum mixer. h). This mixture feeds a briqueter by means of a feeding screw, and it is agglomerated under a pressure greater than 200 bar to form briquettes. These briquettes are then charged in an electric oven with scrap, at a rate of about 1 to 5% by weight of briquettes with respect to the weight of scrap. Heating the briquettes leads to the following reactions:

Reduction of oxides in scrap and the melt:

Fe ₂ O ₃ + 3C → 2Fe + 3CO

ZnO + C → Zn + CO

ZnFe ₂ O ₄ + 4C → Zn + 2Fe + 4CO

PbO + C → Pb + CO

Oxidation of the vaporized metals in the fumes above the melt:

2Fe + 3/2 O ₂ → Fe ₂ O ₃

Zn + ½ O ₂ → ZnO

Pb + ½ O ₂ → PbO

ZnO + Fe ₂ O ₃ → ZnFe ₂ O ₄

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

Table III

The mass balance of the process shows a significant reduction in PAE mass from about 20 kg of PAE per tonne of steel (non-recycle process) to about 9 kg of PAE per tonne of steel (process according to US Pat. 'invention).

Moreover, a cold sulfuric leaching of these PAE leads to a solubilization of 94% to 98% of the zinc: this result shows that a fraction of the iron is present in the form of hematite and not of ferrite, and that the rec- Fe ₂ O3 / ZnO pairing is therefore partial. The explanation for this partial recombination lies in the formation of sodium zincate, with the excess sodium oxide available, according to the following reaction scheme: Na ₂ O + ZnO → Na ₂ ZnO ₂ in competition with the formation of ferrite zinc:

ZnO + Fe ₂ O ₃ → ZnFe ₂ O ₄

The addition of sodium metasilicate during the briquetting therefore promotes the formation of zincate and limits the formation of zinc ferrite.

Thus, besides the neutralization of chlorine and fluorine, briquetting with a source of sodium oxide such as Na ₂ SiO 3 reduces the proportion of insoluble zinc. It may also be noted a reduction in the amount of dioxins present (cyclic organochlorine compounds): in fact, the consumption of chlorine by Na ₂ O limits the available chlorine for the formation of dioxins.

In the same way as the formation of sodium zincates, the formation of zinc silicates (ZnSiO ₄ ) is favored by the introduction of the source of silicon oxide. The recombinations of zinc with silicon have the same advantages as above: limitation of the formation of zinc ferrites, reduction of the insoluble zinc fraction, limitation of the formation of dioxins.

As regards the hydrometallurgical treatment of solubilization of chloride and fluoride salts, it implements the following steps: intensive mixing of PAE (attrition and friction phenomena) in water with mechanical energy flow important, at a solid concentration of about 1000 g / l, for about 30 minutes;

- basic leaching with addition of Na ₂ CO 3 in molar excess, such that the sum of the molar quantities of sodium and potassium contained in the dust, plus those added via the Na ₂ CO ₃ solution, is 30% greater than the sum of the molar quantities of chlorine and fluorine contained in the dust;

filtration under pressure of approximately 15 bar, on a filter press, of the product resulting from the mixing; - washing with water by percolation against the current, at a rate of about 0.5 m ³ of water per tonne of PAE;

- insufflation of air, under pressure of 7 to 15 bars in the chambers of the filter press, to reduce the amount of saline water in the product (interstitial water loaded with chlorides and fluorides), improving the desalination of the product;

- final washing with water by percolation against the current, at a rate of about 0.5 m ³ of water per tonne of PAE;

- insufflation of air, under pressure of 7 to 15 bar, in the chambers of the filter press;

- drying of PAE.

After dissolution, the mass yield is 88% and the composition of the PAE is as follows:

Table IV

This type of product can be upgraded without difficulty because it can be used directly in zinc production plants by sulfur hydrometallurgy.

With regard to the effluents, these have a pH of 8.5 and a composition as follows: Table V

These effluents are discharged at sea after a conventional physico-chemical finishing treatment (coagulation, flocculation, precipitation) to precipitate the metals accidentally leached during the process, and after adjustment of the pH by means of acidic and / or basic solutions usually used for this purpose. use (sulfuric acid, soda).

Claims

1. A method for treating steel mill dust resulting from the melting of scrap metal, comprising steps of:

a) collecting dust by filtration (12) of the gases produced at the outlet of a scrap melting furnace;

b) preparing (14) a mixture comprising: a first fraction of the dusts collected in step a), a carbon source, and a source of sodium oxide;

c) preparing (16) briquettes by agglomeration under pressure of the mixture obtained in step b);

d) briquetting briquettes in the melting furnace receiving scrap, and pyrometallurgical treatment (10) briquettes with scrap; and

e) hydrometallurgical treatment (20-36) for the dissolution of chlorides and fluorides of a second fraction of the dust collected in step a), not used for the mixture of step b),

characterized in that for hydrometallurgical treatment (20-36) of dissolution of chlorides and fluorides of said second fraction of dust:

it is provided in step e) adding (22) a sodium source reagent to the second fraction of the dust.

2. The process of claim 1, wherein the sodium source reagent is selected from sodium hydroxide NaOH and sodium carbonate Na ₂ CO ₃ .

The process of claim 1, wherein the sodium source reagent is added in excess, the molar percentage of the sodium source reagent added in step e) being such that the sum of the molar amounts of sodium and potassium contained therein in the second fraction of the dust and those contained in the sodium source reagent exceeding the sum of the molar quantities of chlorine and fluorine contained in the second fraction of the dust.

4. The process of claim 1, wherein the sum of the molar amounts of sodium and potassium contained in the second fraction of the dust and those contained in the sodium source reagent exceeds 10 to 50%, preferably 20 to 30%. %, the sum of the molar quantities of chlorine and fluorine contained in the second fraction of the dust.

The process of claim 1, wherein the hydrometallurgical treatment of step e) comprises steps of:

· Mixing in the water (20) of the second fraction of the dust;

^• Leaching (s) in the basic phase (22) of the residue obtained with addition of the sodium source reagent;

^• filtration (s) (24) under pressure of the resulting product;

^• washing (26, 30) with water of the residue obtained;

· Insufflation (s) (28, 32) of pressurized air in the filter chambers; and

^• drying (34) of the dust.

6. The process of claim 1, wherein the hydrometallurgical treatment of step e) further comprises a final step of:

^• briquetting (36) without additive, under residual moisture, of the final product obtained.