BE1026928B1 - Hydrometallurgical process for treating dust containing metal oxides from pyrometallurgy - Google Patents

Abstract

A subject of the present invention is therefore a process for the hydrometallurgical treatment of a composition of metal dust derived from at least one electric steelworks comprising the steps of: providing at least one composition, forming at least one heterogeneous mixture having a pH greater than 7 by mixing said at least one composition and at least one inorganic base, separating said at least one heterogeneous mixture into at least one liquid phase and at least one solid phase. Said liquid phase is then treated by different steps using different acidic and basic reagents and at least one compound comprising iron. The method according to the invention makes it possible to generate at least one solid phase which can be upgraded and at least one effluent having a reduced concentration of certain undesirable elements.

Description

“Hydrometallurgical process for treating dust containing metal oxides resulting from pyrometallurgy” Field of the invention The present invention relates to the field of hydrometallurgy and in particular to a hydrometallurgical treatment process generating at least one solid phase having a content reduced in alkali metals, halogens and at least one effluent having a reduced concentration of certain undesirable elements.

State of the art Stainless steel electric steelworks produce stainless steel from scrap metal using electric furnaces such as electric arc furnaces, for example. The scrap used in these steelworks is generally mixtures of stainless steels and different galvanized steels. Consequently, stainless steel electric steelworks generate fumes composed of dust which in general themselves include iron, zinc, nickel, chromium, manganese or even molybdenum and other undesirable elements such as alkali metals and halogens. The dust from these fumes is collected, for example, using bag filters to then be treated. In general, the dust can come, for example, from fine dust entrained by flight or from dust of volatile elements generated during the melting of scrap.

There are processes for treating dust directly from electric steelworks (PAE). Some are hydrometallurgical in nature and others are pyrometallurgical in nature.

Document WO2014044992A1 discloses a process for treating steelworks dust resulting from the smelting of scrap metal. In a first part of the process, part of the dust directly coming from an electric furnace (PAE) is compacted in the form of chips which are then returned to the electric furnace with scrap metal. Another part of the dust (PAE) is treated by a hydrometallurgical process involving, among other things, a basic leaching step with Na2CO3 and washing with water. After the hydrometallurgical treatment, compounds rich in zinc are recovered which can be upgraded.

Dura et al. (Dustra AJB, Paiva PRP and Tavares LM, Alkaline leaching of zinc from electric arc furnace steel dust. Minerals Engineering, April 1, 2006, Vol. 19, No.5, pages 478-485) discloses a hydrometallurgical process for treating PAE comprising among others a leaching step with a concentrated NaOH solution. Different leaching techniques are tested: leaching by stirring, leaching under pressure, leaching after microwave treatment, leaching with stirring by an ultrasonic probe. The zinc recovery rate was 74% at best.

Other pyrometallurgical processes for treating dust directly from electric steelworks (PAE) exist. The dust (PAE) can be recycled in a furnace, generally electric, to produce an Fe-Cr-Ni-Mo alloy which is then reintegrated into the production circuit for stainless steels. Such processes improve the efficiency of stainless steel plants but also generate as by-products dust = metallic compositions (CPM) comprising volatile metal oxides (for example: ZnO, PbO), as well as halogens, alkalis and other metals ( for example: Cr, Mo) in contents which may be much higher than in dust directly resulting from electric steelworks (PAE). The volatile metal oxides are for example recovered by filtration of the fumes, during the remelting of the dust directly coming from stainless steel plants which is rich in Fe, Zn, Cr, Ni, Mo. The fumes are generally characterized by levels of high zinc, alkalis and halogens.

These metal dust compositions (CPM) can be treated by pyrometallurgical processes, such as the Waelz process, for example. However, the presence of halogens and alkalis in high concentrations can cause corrosion and premature wear of the furnaces. As a result, only a part of these compositions (CPM) are treated and a significant part of these compositions (CPM) is landfilled. The part of these compositions (CPM) which can be treated must still be diluted significantly with other zinc compounds low in halogen before treatment. Then, during the Waëlz process, the zinc is volatilized and then re-oxidized in the gas phase to produce Waëlz oxides. The latter, whose halogen contents remain high, cannot be immediately upgraded by electrolysis. They must then undergo an additional roasting step, mixed with blende ores (ZnS) in order to meet market quality criteria.

The significant dilution of the dust compositions (CPM) means that a large part of them cannot be treated in the Waëlz furnaces and must therefore be landfilled. In addition, this pyrometallurgical process requires high temperatures (> 1000 ° C) and is therefore very energy intensive (and therefore very expensive).

In summary, the pyrometallurgical treatments of dust compositions (CPM), in particular by the Waelz process, have many disadvantages: e they generate significant energy costs and an ecological impact, e they do not directly allow the separation between the halogens and the elements. which can be recovered such as zinc and lead, e the alkali metals and halogens present in the dust compositions (CPM) are liable to corrode the furnaces and generate significant maintenance costs, e they produce slag that is difficult to recover and which can release elements such as chromium and molybdenum, e a significant part of the dust compositions (CPM) cannot be treated by the Waelz process and therefore ends up in landfill.

Another process, for example hydrometallurgical, must therefore be considered. However, such a process must meet several criteria. Such a process should allow the treatment of dust compositions (CPM) while making it possible to recover a composition of lead and zinc (CPb-Zn) which can be subsequently upgraded by other treatment routes. Such treatments can be for example: e a pyrometallurgical process, e a pyro and hydrometallurgical process comprising roasting followed by leaching followed by electro-refining, e a hydrometallurgical process comprising a leaching step followed by extraction by zinc solvent followed by electro-refining.

Preferably, the subsequent treatment can be carried out by a pyrometallurgical process, more preferably in a vertical melting furnace of the “Imperial Smelting Furnace” type allowing production of zinc and lead in metallic form. The composition of lead and zinc (CPb-Zn) must therefore have a low alkali and halogen content to prevent corrosion of the furnaces (and peripheral equipment). In addition, the effluents generated by such processes must also include concentrations of elements such as for example Mo, Cr, low enough for example to be treated subsequently by conventional processes.

As the metal dust compositions (CPM) have contents of certain volatile, alkali and halogen oxides generally much higher than in the dust directly from electric steelworks (PAE), the existing hydrometallurgical treatments such as for example that disclosed in WO2014044992A1 are not directly applicable because they do not guarantee the satisfaction of the criteria stated above. There is therefore a need to develop an improved, versatile and economically profitable hydrometallurgical treatment process allowing the treatment of the aforementioned dust compositions (CPM) while ensuring that the products resulting from these processes are easily recoverable and that the effluents generated exhibit high concentrations. in elements such as Zn, Mo, Cr, Pb, F rather weak.

Summary of the Invention The inventors have surprisingly found that it is possible to provide an improved process meeting the aforementioned needs.

The subject of the present invention is therefore a process for the hydrometallurgical treatment of at least one composition of metal dust derived from at least one stainless steel electric steelworks [composition (CPM)] comprising the steps of: a) providing at least one composition (CPM), b) forming at least one heterogeneous mixture (1) having a pH greater than 7 by mixing said at least one composition (CPM) and at least one inorganic base, c) separating said at least one heterogeneous mixture (1) in at least one liquid phase (2) and at least one solid phase (3),

d) forming at least one acid mixture (4) by mixing said at least one liquid phase (2) and at least one inorganic acid, e) forming at least one ferruginous acid mixture (5) by mixing of said at least one acidic mixture (4) with at least one inorganic compound comprising iron, f) formation of at least one ferruginous mixture (6) at pH greater than 6 by mixing said at least one ferruginous acid mixture (5) with at least one inorganic base, g) separation of said ferruginous mixture (6) to obtain at least one effluent and at least one ferruginous solid phase.

Detailed Description of the Invention As used herein and in the claims, the terms "comprising" and "including" are inclusive or open ended and do not exclude other unrequited elements, compositional components or steps of composition. process. The terms "by mixing at least" are also inclusive and do not exclude that other elements or components not recited may be mixed.

The terms "pyrometallurgical process" and "pyrometallurgical treatment" denote respectively a process and a treatment involving techniques for the separation of metals (or metals and impurities) by thermal means. The unit operations undertaken are for example roasting, melting, converting.

The terms “hydrometallurgical process” and “hydrometallurgical treatment” denote respectively a process and a treatment involving techniques for the separation of metals (or of metals and of impurities) by wet process. The unit chemical operations undertaken are, for example, leaching, washing, precipitation, electro-refining.

Composition (CPM) In the context of the present invention, the terms “composition of metal dust derived from at least one stainless steel electric steel plant” may refer for example to a composition of metal dust directly obtained from at least one stainless steel electric steel plant or indirectly from at least one stainless steel electric steelworks.

In the context of the present invention, the terms “stainless steel electric steelworks” refer to a steelworks using an electric furnace, such as for example an electric arc furnace to produce stainless steel. In particular stainless steel produced from scrap which can be mixtures of stainless steels and different galvanized steels.

When the metal dust comes indirectly from at least one stainless steel electric steel plant, the metal dust can for example come from a recycling process for dust directly from at least one stainless steel plant. Preferably, the recycling process can be carried out by a pyrometallurgical treatment, even more preferably a pyrometallurgical treatment using an electric furnace. Said process for recycling dust directly originating from at least one stainless steel plant generally produces a composition of metal dust which may comprise volatile metal oxides (in particular: ZnO, PbO), as well as halogens, alkalis and other metals (in particular: Cr, Mo), in particular in high contents. The volatile metal oxides can be recovered by filtration of fumes generated by said recycling process, for example during the remelting of dust directly from stainless steel plants which are rich in Fe, Zn, Cr, Ni, Mo. Consequently, the terms “metal dust” are not limited to metal dust in oxidation stage 0 but can also include, for example, metals in the form of oxides.

If desired, said step a) of providing said at least one composition (CPM) can comprise at least one process for recycling dust directly from at least one stainless steel electric steel plant. In particular, said recycling process is a pyrometallurgical recycling process, even more particularly a pyrometallurgical recycling process using an electric furnace. Said method for recycling dust directly originating from at least one stainless steel electric steel plant can be a recycling method known to those skilled in the art, such as for example a recycling method disclosed in Antrekowitsch et al. (J. Antrekowitsch, G. Rösler, S. Steinacker, Chemie

Ingenieur Technik, Volume 87, Issue 11, November 2015, pages 1498-

1503.).

If desired, said step a) of providing said at least one composition (CPM) can further comprise a step of collecting at least one composition (CPM) obtained from said process for recycling dust directly originating from at least one steel plant. electric stainless steel.

Advantageously, said at least one composition (CPM) provided in step a) can comprise zinc, preferably a content by weight of zinc greater than or equal to 10% or greater than or equal to 20% or greater than or equal to 25% or greater than or equal to 30% or greater than or equal to 35% in or greater than or equal to 45% by weight of zinc relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the content by weight of zinc is preferably less than or equal to 70% or less than or equal to 60% or less than or equal to 55% or less than or equal to 50% relative to the total dry weight. of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) further comprises lead, preferably a lead content by weight greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 2% or greater than or equal to 3% relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the content by weight of lead is preferably less than or equal to 50% or less than or equal to 30% or less than or equal to 20% or less than or equal to 10% or less than or equal to 6 % or less than or equal to 5% or less than or equal to 4% relative to the total dry weight of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) further comprises at least one alkali, preferably an alkali content greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.3% or greater or equal to 0.5% or greater than or equal to 1% or greater than or equal to 3% or greater than or equal to 5% or greater than or equal to 7% or greater than or equal to 10% by weight of alkali relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the percentage by weight of alkali is preferably less than or equal to 50% or less than or equal to 30% or less than or equal to 20% or less than or equal to 15% or less than or equal to 10 % relative to the total dry weight of said at least one composition (CPM). Preferably, said alkali is chosen from the group of Na, K, Li and their mixtures.

Advantageously, said at least one composition (CPM) further comprises Na, preferably a content by weight of Na greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.3% or greater than or equal to 0.5% or greater than or equal to 2% or greater than or equal to 3% or greater than or equal to 4% or greater than or equal to 5% by weight dry total of said at least one composition (CPM). It is understood that the upper limit of the content by weight of Na is preferably less than or equal to 30% or less than or equal to 20% less than or equal to 10% or less than or equal to 8% or less than or equal to 7% or less than or equal to 6% relative to the total dry weight of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) further comprises K, preferably a content by weight of K greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.3% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 2% or greater than or equal to 2.5% with respect to the total dry weight of said au minus one composition (CPM). It is understood that the upper limit of the content by weight of K is preferably less than or equal to 30% or less than or equal to 20% or less than or equal to 10% or less than or equal to 8% or less than or equal to 7% or less than or equal to 6% relative to the total dry weight of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) further comprises at least one halogen, preferably a content by weight of halogen greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0 , 1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 2% or greater than or equal to 3% or greater than or equal to 4% or greater than or equal to 5% or greater or equal to 6% relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the halogen weight content is preferably less than or equal to 30% less than or equal to 20% or less than or equal to 15% or less than or equal to 14% or less than or equal to 12 % or less than or equal to 11% or less than or equal to 9% relative to the total dry weight of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) further comprises Cl, preferably a content by weight of CI greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 2% or greater than or equal to 3% or greater than or equal to 4% by weight of CI relative to the total dry weight of said au minus one composition (CPM). It is understood that the upper limit of the content by weight of CI is preferably less than or equal to 30% less than or equal to 20% or less than or equal to 10% or less than or equal to 8% or less than or equal to 6% relative to the total dry weight of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) further comprises F, preferably a content by weight of F greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 1.5% or greater than or equal to

2% by weight of F relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the content by weight of F is preferably less than or equal to 30% or less than or equal to 20% or less than or equal to 10% or less than or equal to 8% or less than or equal to 7 % or less than or equal to 6% or less than or equal to 5% relative to the total dry weight of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) comprises at least one additional metal, preferably a content by weight of additional metal greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0, 1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 1.5% relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the content by weight of additional metal is preferably less than or equal to 30% or less than or equal to 20% or less than or equal to 10% or less than or equal to 9% or less than or equal to 8% or less than or equal to 7% relative to the total dry weight of said at least one composition (CPM). Preferably said additional metal is a transition metal, even more preferably said transition metal is chosen from the group consisting of Cr, Fe, Mo and their mixtures.

Advantageously, said at least one composition (CPM) comprises Cr, preferably a content by weight of Cr greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.2% or greater or equal to 0.3% or greater than or equal to 0.3% or greater than or equal to 0.5% greater than or equal to 0.7% or greater than or equal to 0.8% or greater than or equal to 0.9 % or greater than or equal to 1% relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the content by weight of Cr is preferably less than or equal to 30% or less than or equal to 20% or less than or equal to 10% or less than or equal to 8% or less than or equal to 6% or less than or equal to 4% or less than or equal to 3% or less than or equal to 2% relative to the total dry weight of said at least one composition (CPM).

Advantageously, said at least one composition (CPM) comprises Mo, preferably a content by weight of Mo greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater or equal to 0.15% relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the content by weight of Mo is preferably less than or equal to 30% or less than or equal to 20% or less than or equal to 10% or less than or equal to 8% or less than or equal to 6 % or less than or equal to 4% or less than or equal to 5% or less than or equal to 3% or less than or equal to 2% or less than or equal to 1% relative to the total dry weight of said at least one composition (CPM ).

Advantageously, said at least one composition (CPM) comprises Fe, preferably a content by weight of Fe greater than or equal to 0.1% or greater than or equal to 0.3% or greater than or equal to 0.5% or greater or equal to 1% or greater than or equal to 1.5% or greater than or equal to 2% relative to the total dry weight of said at least one composition (CPM). It is understood that the upper limit of the content by weight of Fe is preferably less than or equal to 30% or less than or equal to 20% or less than or equal to 10% or less than or equal to 8% or less than or equal to 7 % or less than or equal to 6% or less than or equal to 5% or less than or equal to 4% relative to the total dry weight of said at least one composition (CPM).

In addition, said at least one composition (CPM) may for example comprise water, in particular a water content greater than 0.1% of water relative to the total dry weight of said at least one composition (CPM). . It is understood that the upper limit of the percentage by weight of water is preferably less than 10% relative to the total dry weight of said at least one composition (CPM). More preferably, said at least one composition (CPM) comprises between 0.1% and 10% water, relative to the total dry weight of said at least one composition (CPM). Advantageously, said at least one composition (CPM) comprises Zn and Pb and at least one element chosen from the group consisting of alkali metals, halogens, Cr, Fe, Mo, and their combinations, moreover Advantageously, said element is selected from the group consisting of selected from the group consisting of Na, K, Cl, F, Cr, Fe, Mo, and combinations thereof.

In a preferred embodiment, said at least one composition (CPM) comprises: e a content by weight of zinc greater than or equal to 30%, e a content by weight of lead greater than or equal to 0.5%, e a content of weight of Cr greater than 0.5%, e a weight content of F greater than 0.1%, and e a weight content of Mo greater than 0.1%, relative to the total dry weight of said at least one composition (CPM). In a preferred embodiment, said at least one composition (CPM) comprises: e between 30% and 55% by weight of zinc, e between 0.5% and 6% by weight of lead, e between 1% and 6% by weight of F, e between 0.5% and 3% by weight of Cr, and e between 0.1% and 2% by weight of Mo, relative to the total dry weight of said at least one composition (CPM). In a preferred embodiment, said at least one composition (CPM) comprises: e at least 30% by weight of zinc, e at least 0.5% by weight of lead,

e at least at least 0.5% by weight of Na, e at least 0.5% by weight of K, e at least 2% by weight of CI, e at least 1% by weight of F, e at least 0 , 5% by weight of Cr, e at least 0.1% by weight of Mo, and e at least 0.5% by weight of Fe, relative to the total dry weight of said at least one composition (CPM).

In another preferred embodiment, said at least one composition (CPM) comprises: e between 30% and 55% by weight of zinc, e between 0.5% and 6% by weight of lead, e between 0.5% and 10% by weight of Na, e between 0.5% and 10% by weight of K, e between 2% and 8% by weight of CI, e between 1% and 6% by weight of F, e between 0, 5% and 3% by weight of Cr, e between 0.1% and 2% by weight of Mo, and e between 0.5% and 6% by weight of Fe, relative to the total dry weight of said at least one composition (CPM). Step b) As explained, said at least one composition (CPM) and at least one inorganic base are mixed to form at least one heterogeneous mixture (1) having a pH greater than 7.

Advantageously, said at least one inorganic base has the formula MxAy in which M is an alkaline or alkaline-earth cation, preferably chosen from the group consisting of Li *, Na *, K +, Mg **, Ca? + Ba? and their combinations and A is an anion, preferably selected from the group consisting of OH, COs *, and their combinations, the coefficients x and y can each take the values 1 or 2. Obviously, the value of the coefficients depends on the cation M and of the anion A.

Even more preferably, said at least one inorganic base is chosen from the group consisting of LIOH, NaOH, KOH, Ca (OH) z, Mg (OH) 2, Li2CO3, Na2CO3, K2CO3, CaCO3, MgCO3, BaCO: and their mixtures. Even more preferably, said at least one inorganic base is chosen from the group consisting of Na2CO3, NaOH, KOH, K2CO3, Ca (OH) 2 and their mixtures.

It appeared that excellent results were obtained when said at least one inorganic base is chosen from the group consisting of Na2COs, NaOH, KOH, K2CO3, Ca (OH) 2 and their mixtures.

Advantageously, said step b) of forming said at least one heterogeneous mixture (1) is carried out by mixing said at least one composition (CPM) and an excess of said at least one inorganic base relative to said at least one composition (CPM).

Advantageously, said step b) of forming said at least one heterogeneous mixture (1) is carried out by mixing said at least one composition (CPM) and at least 5 kg, preferably at least 10 kg, more preferably with less 20 kg, even more preferably at least 30 kg of an inorganic base per dry ton of said at least one composition (CPM).

Advantageously, said step b) of forming said at least one heterogeneous mixture (1) is carried out by mixing said at least one composition (CPM) and at most 400 kg, preferably at most 350 kg, more preferably at more 300 Kg even more preferably at most 200 Kg, even more preferably at most 100 Kg, even more preferably at most 80 Kg of an inorganic base, per dry ton of said at least one composition (CPM) .

Particularly excellent results have been obtained when said step b) of forming said at least one heterogeneous mixture (1) is carried out by mixing said at least one composition (CPM) and between 30 kg and 80 kg of an inorganic base per dry ton of said at least one composition (CPM). Advantageously, said at least one heterogeneous mixture (1) having a pH greater than 7 is produced by mixing said at least one composition (CPM) and at least one inorganic base at a temperature greater than 0 ° C, preferably greater than at 5 ° C, more preferably greater than 10 ° C, even more preferably greater than 15 ° C.

Advantageously, said at least one heterogeneous mixture (1) having a pH greater than 7 is produced by mixing said at least one composition (CPM) and at least one inorganic base at a temperature below 100 ° C, preferably below at 80 ° C, more preferably less than 55 ° C.

Advantageously, said at least one heterogeneous mixture (1) having a pH greater than 7 is produced by mixing said at least one composition (CPM) and at least one inorganic base at a temperature between 15 and 55 ° C.

Advantageously, said at least one heterogeneous mixture (1) having a pH greater than 7 is produced by mixing said at least one composition (CPM) and at least one inorganic base for a period greater than 10 minutes, preferably greater than 20 minutes, more preferably greater than 30 minutes, even more preferably greater than 45 minutes.

Advantageously, said at least one heterogeneous mixture (1) having a pH greater than 7 is produced by mixing said at least one composition (CPM) and at least one inorganic base for a period of less than 10 hours, preferably less than 5 hours, more preferably less than 2 hours, more preferably less than 1.5 hours.

Advantageously, said at least one heterogeneous mixture (1) having a pH greater than 7 is produced by mixing said at least one composition (CPM) and at least one inorganic base for a period of between 45 minutes and 1.5 hours .

Said at least one composition (CPM) and said at least one base are preferably mixed in the presence of an aqueous solution to form said at least one heterogeneous mixture (1).

Therefore, said step b) of forming at least one heterogeneous mixture (1) is preferably carried out by mixing said at least composition (CPM), at least one aqueous solution and at least one inorganic base.

In the context of the present invention, the term "aqueous solution" can refer to water, deionized water, salt water or any aqueous solution of inorganic salts.

Those skilled in the art will mix said at least one composition (CPM), said at least one aqueous solution and said at least one inorganic base in such a way that the pH of said at least one heterogeneous mixture (1) is greater than 7 .

Advantageously, said at least one heterogeneous mixture (1) has a pH greater than 8, preferably greater than 9, more preferably greater than 10.

Advantageously, said at least one heterogeneous mixture (1) has a pH less than 13.5, preferably less than 13.

Advantageously, said at least one heterogeneous mixture (1) has a pH of between 10 and 13.

The sequence of mixing the components, i.e. said at least one composition (CPM), said at least one aqueous solution and said at least one inorganic base can be as desired. Optionally, one or two or more components can be premixed, or all of the components can be mixed together. For example said at least one composition (CPM) and said at least one aqueous solution can be mixed in a step followed by another step in which said at least one inorganic base is added. In another example, said at least one inorganic base and said at least one aqueous solution are mixed in a step followed by another step in which said at least one composition (CPM) is added. In yet another example, said at least one inorganic base and said at least one composition (CPM) are mixed in a first step followed by a second step in which said at least one aqueous solution is added. Preferably, said at least one composition (CPM) and said at least one aqueous solution are mixed in a step followed by another step in which said at least one inorganic base is added.

Particularly, said step b) of forming at least one heterogeneous mixture (1) can comprise: e a step bi) of mixing said at least one composition (CPM) in at least one aqueous solution to obtain at least one pulp, e a step b2) of adding said at least one inorganic base to said pulp obtained in step b1).

In the context of the present invention, the pulp is a term which designates the mixture obtained by mixing said at least one composition (CPM) and said aqueous solution.

Advantageously, said at least one pulp has a concentration of said composition (CPM) greater than 50 g / L, preferably greater than 100 g / L, more preferably greater than 150 g / L.

Advantageously, said at least one pulp has a concentration of said composition (CPM) of less than 600 g / L, preferably less than 500 g / L, even more preferably less than 450 g / L.

Excellent results have been obtained when said at least one pulp has a concentration of said composition (CPM) of between 150 and 450 g / L.

Stepec) In the context of the present invention, said step c) of separating said at least one heterogeneous mixture (1) into at least one liquid phase (2) and at least one solid phase (3) can be carried out by any which means known to those skilled in the art, for example by filtration, or alternatively decantation or centrifugation.

Advantageously, said step c) of separating said at least one heterogeneous mixture (1) comprises a step of filtering said at least one heterogeneous mixture (1).

Said at least one solid phase (3) of step c) can undergo a washing step, preferably a washing step with an aqueous solution. For example, said washing step may consist of adding water to said at least one solid phase followed by filtration. Advantageously, the aqueous solution having served as washing can be recovered after filtration and forms part of said at least one liquid phase (2).

It was surprisingly observed that the succession of steps a), b) and c) according to the invention makes it possible to recover a solid phase (3) which comprises high concentrations of zinc and lead, and low of halogens and alkalis. . Thanks to the process according to the invention, the content by weight of alkali and halogen in said at least one solid phase (3) decreases to more than 50%, preferably more than 60%, more preferably more than 70 % based on the content by weight of alkalis and halogens in said at least one composition (CPM). In particular, the content by weight of chlorine could decrease by more than 50%, preferably more than 70% preferably more than 90%, more preferably more than 98% relative to the content by weight of chloredans in said at least one composition (CPM). In particular, the content by weight of fluorine could decrease by more than 50%, preferably more than 60% preferably more than 70% relative to the content by weight of fluorine in said at least one composition (CPM). content by weight of Zn and Pb in said at least one solid phase (3) can increase from 5 to 40%, preferably from 15 to 30% relative to the content by weight of Zn and Pb in said at least a composition (CPM). Such concentrations in said at least one solid phase (3) allow the subsequent use of said at least one solid phase (3) in a pyrometallurgical treatment process, for example in a vertical melting furnace of the "Imperial Smelting Furnace" type. In general, in such a furnace of the blast furnace type, the zinc is reduced and then volatilized in the form of a gas. This gas is captured at the top of the furnace and passes through a spray chamber of droplets of molten lead at a temperature of about 550. ° C, the zinc is condensed there as a mixed ZnPb alloy. Zinc and lead are then separated by liquation. The zinc then "floats" above the lead and can be separated by density to be cast into ingots. The low concentrations of halogens and alkalis make it possible to limit as much as possible the deterioration of such furnaces and their peripheral equipment.

In general, the liquid phase (2) obtained in step c) contains concentrations of elements such as Zn, Pb, F, Cr and Mo which render its treatment in purification plants with conventional physicochemical treatment, difficult, if not impossible. The liquid phase (2) must therefore still be treated.

For example, said at least one liquid phase (2) may comprise a concentration greater than 0.05 g / L of Mo, preferably greater than 0.1 g / L, even more preferably greater than 0.2 g / L. The Mo concentration can also be less than 3 g / L, preferably 2 g / L, more preferably less than 1 g / L, more preferably less than 0.6 g / L.

The liquid phase (2) can also comprise a concentration greater than 0.05 g / L of Cr, preferably greater than 0.07 g / L, even more preferably greater than 0.1 g / L.

The Cr concentration can also be less than 3 g / L, preferably 2 g / L, more preferably less than 1 g / L.

The liquid phase (2) can also comprise a concentration greater than 0.05 g / L of Pb, preferably greater than 0.07 g / L, even more preferably greater than 0.1 g / L.

The Pb concentration can also be less than 10 g / L, preferably 5 g / L, more preferably less than 1 g / L.

The liquid phase (2) can also comprise a concentration greater than 0.05 g / L of Zn, preferably greater than 0.07 g / L, even more preferably greater than 0.1 g / L.

The Zn concentration can also be less than 5 g / L, preferably 1.5 g / L, more preferably less than 0.5 g / L.

The liquid phase (2) can also comprise a concentration greater than 0.1 g / L of F, preferably greater than 0.3 g / L, even more preferably greater than 0.5 g / L.

The concentration of F can also be less than 15 g / L, preferably 10 g / L, more preferably less than 8 g / L.

Step d) As explained, in said step d) said at least one liquid phase (2) and at least one inorganic acid are mixed to form at least one acid mixture (4). The sequence of mixing the components, i.e. said at least one inorganic acid and said at least one liquid phase (2) can be as desired.

Optionally, one or two or more components can be at least partially premixed, or all of the components can be mixed together.

Preferably, said at least one inorganic acid is added to said at least one liquid phase (2).

Advantageously, said at least one inorganic acid is a strong inorganic acid. Preferably, said at least one strong inorganic acid is an acid selected from the group consisting of HCl, H2SO4 and mixtures thereof.

For example, said at least one inorganic acid can be in dilute or concentrated aqueous or non-aqueous solution.

Advantageously, said at least one inorganic acid is sulfuric acid.

In general, said at least one acidic mixture obtained in step d) has a pH less than 7, preferably less than or equal to 6.

If desired, said at least one acid mixture obtained in step d) can have a pH greater than or equal to 1, preferably greater than or equal to 2. Step e) As explained, in said step e), said at least one acid mixture (4) and at least one iron compound are mixed to form at least one ferruginous acid mixture (5).

The sequence of mixing the components can be as desired. Optionally, one or two or more components can be premixed, or all of the components can be mixed together. Preferably, said compound comprising iron is added to said acid mixture (4). More preferably, said compound comprising iron is added in excess to said acid mixture (4) relative to at least one element present in said at least one composition (CPM) and chosen from the group consisting of Zn, Pb, Na, K, CI, F, Cr, Fe, Mo and their mixtures.

Advantageously, said compound comprising iron can be chosen from the group consisting of iron oxides, iron sulphates, iron chlorides and their combinations.

Advantageously, said compound comprising iron is chosen from the group consisting of Fe2O3, FeCls, FeSO4, FeO, Fez (SO4) s and their mixtures. Step f) As explained, in said step f) said at least one ferruginous acid mixture (5) and at least one inorganic base are mixed to form said at least one ferruginous mixture (6).

The sequence of mixing the components can be as desired. Optionally, one or two or more components can be premixed, or all of the components can be mixed together. Preferably, said at least one inorganic base is added to said at least one ferruginous acid mixture (5).

If desired, said ferruginous mixture (6) can have a pH less than or equal to 10, preferably less than or equal to 9 ,.

Advantageously, in step f) said at least one inorganic base is chosen from the group consisting of LIOH, NaOH, KOH, Ca (OH) 2, CaO, Mg (OH) 2, Li2CO3, Na2CO3, K2CO3, CaCO3, BaCOs , MgCO3, and combinations thereof.

It appeared that excellent results were obtained when in step f) said at least one inorganic base is selected from the group consisting of KOH, K2CO3, NaOH, CaO, Na2CO3, Ca (OH) 2 and mixtures thereof.

In general, the purpose of using said at least one inorganic base is to raise the pH of said at least one ferruginous acid mixture (5) above 5 to form said at least one ferruginous mixture (6). This step makes it possible to precipitate, at least in part, undesirable elements such as F, Mo or Cr which were originally present in said at least one liquid phase (and therefore also present in said at least one ferruginous acid mixture (5). Step g) In the context of the present invention, said step g) of separating said at least one ferruginous mixture (6) to obtain at least one effluent and at least one solid ferruginous phase can be carried out by any means known to man. professional, for example by filtration, or decantation or centrifugation.

Advantageously, said step g) of separating said at least one ferruginous mixture (6) comprises a step of filtering said at least one ferruginous mixture (6). Said at least one ferruginous solid phase of step g) can undergo a washing step, preferably a washing step with an aqueous solution. For example, said washing step may consist of adding water to said at least one ferruginous solid phase followed by filtration. The aqueous solution which was used for washing can be recovered after filtration and forms part of said at least one effluent. It has been observed, surprisingly, that the succession of the steps of the process according to the invention makes it possible to recover at least one effluent which can meet environmental standards. Indeed, the process according to the invention makes it possible, for example, to obtain at least one effluent having rather low concentrations of elements such as, for example, Zn, Pb, Cr, F or Mo. Consequently, said at least one effluent could be treated in purification stations with conventional physico-chemical treatment if desired. The steps of the process according to the invention and in particular steps e), f) and g) make it possible to precipitate the aforementioned elements. By comparing with the concentrations of the elements present in said at least one liquid phase (2) and in said at least one effluent, high precipitation yields could be calculated.

In the context of the present invention, each preferred, particular or advantageous variant of any embodiment of the invention can be applied to any other embodiment of the invention.

Example 1 In a first step, a composition of metal dust (CPM) resulting from a recycling process carried out by pyrometallurgical treatment using an electric furnace, of dust directly resulting from a stainless steel plant is obtained, it comprises the elements specified in the table 1.

Table 1 Content (%) by weight relative to the total dry weight of the composition (CPM)

D The composition (CPM) is then mixed with water to form a pulp with a concentration between 1000kg / m * and 1300 kg / m °. After a few minutes, the pulp is homogeneous thanks to continuous agitation.

The inorganic base is present in an amount of between 200 and 300 kg per dry ton of composition (CPM), so as to obtain a heterogeneous mixture having a pH of between 10.5 and 14. This is then filtered.

A liquid phase and a solid phase are recovered.

The solid phase is then washed with water.

The water recovered during this filtration is part of said liquid phase.

The solid phase which is largely composed of zinc and lead comprises the elements specified in Table 2. Table 2 Element of the solid phase Content (%) by weight with respect to eg As illustrated, the concentration of Na, K, Cl and F is particularly low, which makes it possible to upgrade this solid phase by treatment in an Imperial Smelting Furnace so as to produce Zn and metallic Pb.

Then sulfuric acid is added to said liquid phase to form an acidic mixture.

Then a compound comprising iron is added to the acidic mixture to form an acidic ferruginous mixture.

The compound comprising iron is selected from the group consisting of Fe2O3, FeCls, FeSO4, FeO, Fez (SO4) s and mixtures thereof.

Then, an inorganic base is added so as to obtain a ferruginous mixture with a pH greater than 6. The ferruginous mixture thus obtained is then filtered.

The collected effluent exhibits precipitation yields of certain metals and halogens as illustrated below in Table 3. The yields are calculated by comparing the amounts of elements present in the liquid phase and the effluent. Table 3

PERFORMANCE ELEMENT

PRECIPITATION Me 1 RO | Hey

SE Calculation of the yield according to: n = "Cinitial (X) xVinitial- Cfinal (X) xVfinal" / "Cinitial (X) xVinitial" = "Cinitial (X) -Cfinal (X)" / "Cinitial (X)" car Vinitial = Vfinal Vinitial = volume of the liquid phase Vfinal = volume of the effluent X being the element in solution in question. Cinitial (X) = Concentration of X in the liquid phase Cfinal (X) = Concentration of x in the effluent The element concentrations being measured by ICP (ICAP 6000 Series - Thermo Scientific). As can be seen, the precipitation yields of Zn, Pb, F, Cr and Mo are particularly high. This means that the concentrations of Zn, Pb, F, Cr and Mo in the effluent are particularly low, which makes it possible to treat said effluent by conventional routes.

Comparative Example A liquid phase having the composition specified in Example 1 is recovered.

Then sulfuric acid is added to said liquid phase.

Then a compound comprising iron is added to the acidic mixture to form an acidic ferruginous mixture. The compound comprising iron is selected from the group consisting of Fe2O3, FeCls, FeSO4, FeO, Fez (SO4) s and mixtures thereof. The pH of the ferruginous mixture obtained is less than 4.

The acidic ferruginous mixture thus obtained is then filtered. The collected effluent exhibits precipitation yields of certain metals and halogens as illustrated below in Table 4.

Table 4 precipitation The yield calculation was carried out as in Example 1.

As can be seen, the precipitation yields of Cr, Zn and Mo are lower than the precipitation yield presented in Example 1. This shows that the effluent comprises higher concentrations of Zn, Cr and Mo, not compatible with a wastewater treatment plant with conventional physico-chemical treatment.

Claims (21)

1. A method of hydrometallurgical treatment of at least one composition of metal dust derived from at least one stainless steel electric steel plant [composition (CPM)] comprising the steps of: a) providing at least one composition (CPM), b) forming at least one heterogeneous mixture (1) having a pH greater than 7 by mixing said at least one composition (CPM) and at least one inorganic base, C) separating said at least one heterogeneous mixture (1) into at least one liquid phase (2) and at least one solid phase (3), d) formation of at least one acid mixture (4) by mixing said at least one liquid phase (2) and at least one acid inorganic, e) forming at least one ferruginous acid mixture (5) by mixing said at least one acidic mixture (4) with at least one inorganic compound comprising iron, f) forming at least one ferruginous mixture (6) at pH greater than 6 by mixing said at least one ferruginous acid mixture (5) with at least one inorganic base, g) separati of said ferruginous mixture (6) to obtain at least one effluent and at least one ferruginous solid phase. 2. The method of claim 1, wherein said at least one composition (CPM) comprises a content by weight of zinc greater than 30% or greater than or equal to 35% or greater than or equal to 45% by weight of zinc relative to to the total dry weight of said at least one composition (CPM). 3. Method according to claim 1 or claim 2, wherein said at least one composition (CPM) further comprises lead, preferably a lead content by weight greater than or equal to 0.01% or greater than or equal to 0, 05% or greater than or equal to 0.1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 2% or greater than or equal to 3% with respect to the total dry weight of said au minus one composition (CPM). 4. Method according to any one of the preceding claims, wherein said at least one composition (CPM) further comprises Na, preferably a content by weight of Na greater than or equal to 0.01% or greater than or equal to 0 , 05% or greater than or equal to 0.1% or greater than or equal to 0.3% or greater than or equal to 0.5% or greater than or equal to 2% or greater than or equal to 3% or greater than or equal to 4 % or greater than or equal to 5% relative to the total dry weight of said at least one composition (CPM). 5. Method according to any one of the preceding claims, wherein said at least one composition (CPM) further comprises K, preferably a content by weight of K greater than or equal to 0.01% or greater than or equal to 0 , 05% or greater than or equal to 0.1% or greater than or equal to 0.3% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 2% or greater than or equal to 2 5% relative to the total dry weight of said at least one composition (CPM). 6. Method according to any one of the preceding claims, wherein said at least one composition (CPM) further comprises CI, preferably a content by weight of CI greater than or equal to 0.01% or greater than or equal to 0. , 05% or greater than or equal to 0.1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 2% or greater than or equal to 3% or greater than or equal to 4% in weight of CI relative to the total dry weight of said at least one composition (CPM). 7. A method according to any one of the preceding claims, wherein said at least one composition (CPM) further comprises F, preferably a content by weight of F greater than or equal to 0.01% or greater than or equal to 0.05% or greater than or equal to 0.1% or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 1.5% or greater than or equal to 2% by weight of F relative to the total dry weight of said at least one composition (CPM). 8. Method according to any one of the preceding claims, wherein said at least one composition (CPM) comprises Cr, preferably a content by weight of Cr greater than or equal to 0.05% or greater than or equal to 0.1. % or greater than or equal to 0.2% or greater than or equal to 0.3% or greater than or equal to 0.3% or greater than or equal to 0.5% greater than or equal to 0.7% or greater than or equal to 0.8% or greater than or equal to 0.9% or greater than or equal to 1% relative to the total dry weight of said at least one composition (CPM). 9. The method according to any one of the preceding claims, wherein said at least one composition (CPM) comprises Mo, preferably a content by weight of Mo greater than or equal to 0.01% or greater than or equal to 0.05. % or greater than or equal to 0.1% or greater than or equal to 0.15% relative to the total dry weight of said at least one composition (CPM). 10.Process according to any one of the preceding claims, wherein said at least one composition (CPM) comprises Fe, preferably a content by weight of Fe greater than or equal to 0.1% or greater than or equal to 0.3. % or greater than or equal to 0.5% or greater than or equal to 1% or greater than or equal to 1.5% or greater than or equal to 2% relative to the total dry weight of said at least one composition (CPM). 11.Process according to any one of the preceding claims, wherein said at least one composition (CPM) comprises: e between 30% and 55% by weight of zinc, e between 0.5% and 6% by weight of lead, e between 0.5% and 10% by weight of Na, e between 0.5% and 10% by weight of weight of K, e between 2% and 8% by weight of CI, e between 1% and 6% by weight of F, e between 0.5% and 3% by weight of Cr, e between 0.1% and 2 % by weight of Mo, and e between 0.5% and 6% by weight of Fe, relative to the total dry weight of said at least one composition (CPM). 12. A process according to any preceding claim, wherein said compound comprising iron may be selected from the group consisting of iron oxides, iron sulfates, iron chlorides, and combinations thereof. 13.Process according to any one of the preceding claims wherein, in said step f) said at least one inorganic base is selected from the group consisting of LIOH, NaOH, KOH, Ca (OH) a, CaO, Mg (OH) a, Li2CO3, Na2CO3, K2CO3, CaCOs, BaCOs, MgCOs, and their combinations. 14.Process according to any one of the preceding claims, wherein in step b) said at least one inorganic base has the formula MxAy in which M is an alkaline or alkaline-earth cation, preferably selected from the group consisting of Li ”, Na *, K *, Mg, Ca” Ba ”and their combinations and A is an anion, preferably selected from the group consisting of OH, CO3%, and their combinations, x and y being coefficients each taking the values 1 or 2. 15.Process according to any one of the preceding claims, wherein said step b) of forming said at least one heterogeneous mixture (1) is carried out by mixing said at least one composition (CPM) and an excess of said at least one composition (CPM). minus one inorganic base relative to said at least one composition (CPM). 16.Process according to any one of the preceding claims, wherein said step b) of forming said at least one heterogeneous mixture (1) is carried out by mixing said at least one composition (CPM) and at least 5 kg, preferably at least 10 kg, more preferably at least 20 kg, even more preferably at least 30 kg of an inorganic base per dry ton of said at least one composition (CPM). 17. A process according to any one of the preceding claims, wherein said at least one heterogeneous mixture (1) has a pH greater than 8. 18. A method according to any one of the preceding claims, wherein said at least one heterogeneous mixture (1) has a pH greater than 9. 19. A method according to any one of the preceding claims, wherein said at least one heterogeneous mixture (1) has a pH greater than 10. 20.Process according to any one of the preceding claims, wherein said step b) of forming at least one heterogeneous mixture (1) may comprise a step b1) of mixing said at least one composition (CPM) in at least an aqueous solution to obtain at least one pulp, a step b2) of adding said at least one inorganic base to said pulp obtained in step b1). 21.Process according to any one of the preceding claims, wherein said at least one pulp has a concentration of said composition (CPM) greater than 50 g / L.