FR3150512A1 - PROCESS FOR REUSING A CYANIDED REFRACTORY PRODUCT - Google Patents

Abstract

The invention relates to a method for reusing a used refractory product initially placed, in a service position, within a thermal installation and contaminated by an alkaline cyanide compound, said method comprising the following successive steps: a) - extraction of the used refractory product from the service position, - particle size reduction of the used refractory product so as to obtain a particulate mixture; b) before said extraction and/or during said extraction, and/or between said extraction and said particle size reduction and/or during said particle size reduction of step a) and/or after step a), contacting the particulate mixture with a solution comprising a thiosulfate compound, or "thiosulfate solution", so as to obtain a two-phase mixture; e) separation of the liquid phase of the two-phase mixture and use of the particulate mixture to manufacture a new refractory product. No abstract figure

Description

PROCESS FOR REUSING A CYANIDED REFRACTORY PRODUCT

The invention relates to a method for reusing a refractory product contaminated by an alkaline cyanide compound.

State of the art

The reuse of used refractory products present in a thermal installation, for example in a furnace, generally consists of grinding them in the form of a granular mixture, then reusing this granular mixture in other applications (metallurgy, cement works in particular) but also to manufacture new refractory products, preferably for the same application.

Used refractory products, in particular refractory products containing carbon, in particular containing SiC, or containing nitrogen, in particular a nitrogen matrix, for example SiAlON, may however be contaminated by alkaline cyanide compounds. This contamination changes the chemical composition and does not allow, by simple grinding, to obtain a secondary material suitable for manufacturing new refractory products.

Furthermore, a reduction in the content of alkali cyanide compounds may require thermal or chemical treatment leading to a modification of the chemical composition or crystallographic structure of the refractory products. Such a reduction in the content of alkali cyanide compounds may thus lead to products unsuitable for the manufacture of new refractory products.

Furthermore, treatment to reduce the content of alkaline cyanide compounds must not lead to the generation of new compounds that may be harmful, particularly if the particulate mixture obtained is intended to be reused at high temperatures. In particular, the generation of hydrocyanic acid vapours must be avoided.

Finally, used refractory products contaminated with alkaline cyanide compounds are often also contaminated with other compounds that also need to be extracted and/or neutralised, such as fluorides, silica or heavy metals. The succession of treatments requires special precautions so that a first treatment is not detrimental to the following treatment. This makes the process complex.

Processes for destroying cyanide compounds by heat treatment are known. US5470559 describes such a treatment which requires grinding, contact with a soda solution, then heating between 160 and 220°C under pressure. This process presents health risks (release of toxic gas) and is difficult to implement industrially on large volumes.

Cyanide compounds can also be destroyed by chemical treatment using calcium or sodium hypochlorite, which is highly oxidizing and potentially dangerous, as described in US6696617B1 or hydrogen peroxide followed by washing with soda as described in EP3868907A1.

GB2056425 describes a treatment with lime and soda resulting in a dilute liquor comprising fluorides, soda and cyanides.

WO94/02263 describes pre-calcination of used refractory product, which can produce harmful fluorinated vapours.

Despite these numerous solutions, there is still a need for a reuse process that is simpler, risk-free and suitable for different refractory products contaminated with an alkaline cyanide compound.

The present invention aims to at least partially satisfy this need.

According to a first main aspect, the invention relates to a method for treating a refractory product contaminated by an alkaline cyanide compound, said method comprising contacting the used refractory product with a solution comprising a thiosulfate compound.

The inventors have in fact discovered that simply bringing a refractory product into contact, for example by painting, dipping or spraying, with a solution containing a thiosulfate compound considerably reduces the risk of cyanide gas emission, in particular when handling the refractory product, in particular with a view to its subsequent treatment.

The neutralization of cyanide compounds present in the blood of patients, therefore in a liquid medium, at a substantially neutral pH and at approximately 37°C, by the injection of a thiosulfate compound is known to doctors. The inventors were however surprised to note that a thiosulfate compound had an effect when the alkaline cyanide compound is in the solid phase, and that this effect was present at higher pHs than in the blood, preferably at pHs above 8, and even 9, in particular in the presence of the basic solution. They were also surprised by the intensity of this effect, sufficient for an industrial application of the process.

Furthermore, the inventors have discovered that the neutralization of cyanide compounds of a refractory product with a thiosulfate solution can be catalyzed by the addition of a metal such as copper, preferably in organometallic form, for example in the form of copper sulfate. The molar content of catalyst is preferably greater than 5%, preferably greater than 10%, preferably greater than 20% and/or less than 50%, or even less than 40%, as a percentage based on the number of moles of thiosulfate compound.

The invention relates in particular to a method for treating a refractory product, preferably initially placed, in a service position, within a thermal installation, contaminated by an alkaline cyanide compound, said method comprising the following step a'):
- optionally, extraction of the refractory product out of the service position;
- preferably, bringing the refractory product into contact with a solution comprising a thiosulfate compound, before, during or after said extraction;
- optionally, transport, before or after said granulometric reduction, of the refractory product to a remote treatment center, for example more than 1 km from the thermal installation;
- preferably, particle size reduction of the refractory product so as to obtain a particulate mixture and preferably, contacting the particulate mixture with a solution comprising a thiosulfate compound, identical to or different from the solution optionally contacted with the refractory product before particle size reduction.

The operations of a method according to the first main aspect may comprise one or more of the characteristics, optional or not, of the corresponding operations of a method according to the second main aspect described below.

According to a second main aspect, the invention relates to a method for reusing a used refractory product initially placed, in a service position, within a thermal installation and contaminated by an alkaline cyanide compound, said method comprising the following successive steps:
has)
- preferably, contacting the used refractory product in the service position, with a first solution comprising a thiosulfate compound, or first “thiosulfate solution”,
- extraction of the used refractory product out of the service position,
- preferably, during and/or after said extraction, bringing the used refractory product into contact with a second thiosulfate solution, identical to or different from the first thiosulfate solution, and
- optionally, transport of the used refractory product to a remote treatment centre, for example more than 1 km from the thermal installation;
- particle size reduction of the used refractory product so as to obtain a particulate mixture, said transport possibly being before or after the particle size reduction;
b) after step a) and/or during the particle size reduction operation of step a), bringing the particulate mixture into contact with a third thiosulfate solution, identical to or different from the first and second thiosulfate solutions, so as to obtain a two-phase mixture;
e) separation of the liquid phase from the two-phase mixture and use of the particulate mixture, preferably for manufacturing a new refractory product.

Step b) may be carried out before said extraction and/or during said extraction, and/or between said extraction and said particle size reduction and/or during said particle size reduction of step a) and/or after step a).

The features of the two main embodiments can be combined.

In the remainder of the description, a solution comprising a thiosulfate compound may also be referred to as a “thiosulfate solution”. In a method according to the invention, first, second and third thiosulfate solutions, which may be identical or different, may be used. Unless otherwise indicated, for the sake of clarity, “thiosulfate solution” designates, depending on the context, the first, second or third thiosulfate solution.

In a preferred embodiment, the two main aspects of the invention are combined, step a) comprising one or more of the features of step a'). In particular, preferably, the reuse process comprises , before, during or after said extraction, contacting the used refractory product with a thiosulfate solution before said particle size reduction of the used refractory product so as to obtain a particulate mixture.

The thiosulfate solutions used in step a’) and then in step b) may be identical or different.

However, treatment of the refractory product before particle size reduction with a thiosulfate solution is optional.

In step a) and/or step b), the used refractory product or the particulate mixture, respectively, is preferably kept in contact with a thiosulfate solution until the cyanide (CN ^- ) content, measured by a leaching test according to ISO 14403-2:2012 Part 2, is less than 25 mg/litre, or even less than 20 mg/litre.

In one embodiment, step b) is simultaneous with the particle size reduction of step a). Advantageously, the risk of cyanide gas emission during the particle size reduction is reduced.

In a particularly advantageous embodiment, the method further comprises the following step:
c) after step a), subsequently, simultaneously or prior to step b), preferably subsequently to step b), bringing the particulate mixture into contact with a solution with a pH greater than 8, greater than 9, greater than 10, greater than 11, greater than 12, and even greater than 13, or “basic solution”.

Step c) can be done before step e).

The thiosulfate compound may be in particular an alkali thiosulfate or an alkaline earth thiosulfate.

The basic solution may be a solution added to the particulate mixture after separation of the thiosulfate solution, for example an aqueous sodium hydroxide solution. Preferably, in the absence of separation, the basic solution is a mixture between the thiosulfate solution (not separated) and an added base, for example NaOH, in an amount and at a concentration suitable to obtain the desired pH.

Unlike the methods of the prior art, the method according to the invention then combines contact with at least one thiosulfate solution and contact with the basic solution, preferably resulting from the addition of an aqueous solution comprising sodium hydroxide (NaOH) and/or lime (Ca(OH) ₂ ). Unexpectedly, and contrary to their initial fear, the inventors found that contact with the basic solution does not produce cyanide beyond 25 mg/liter, in particular more sodium cyanide when the basic solution is sodium hydroxide.

Unexpectedly, the alkaline cyanide compounds initially present in the used refractory product are neutralizable by the thiosulfate solution, and remain neutralized, including at a pH greater than 8, greater than 9, greater than 10, greater than 11, greater than 12, and even greater than 13.

Treatment with a basic solution advantageously allows the solubilization and then neutralization of fluorine compounds, preferably in the form of sodium fluoride in the case of a basic soda solution or preferably in the form of calcium fluoride in the case of a basic lime solution. It also allows leach the silica, especially with a solution of alkali hydroxide.

The inventors have also found, without being able to explain it theoretically, that the two-phase mixture containing the particulate mixture and the basic solution can be heated, in particular up to a temperature of 80°C and at a pressure of 0.1 MPa without emitting hydrocyanic acid vapours in harmful quantities. It can thus in particular be handled without risk to the operators. After separation of the liquid phase, the particulate mixture no longer emits cyanides and can be reused to manufacture a new sintered refractory product, preferably after additional treatment to treat other possible contaminants.

The inventors finally discovered that the basic solution could be brought into contact with the particulate mixture even in the presence of the thiocyanates resulting from the previous steps. Advantageously, the thiosulfate compound can be mixed with the basic solution and before being brought into contact with the refractory product or the particulate mixture. Steps b) and c) are carried out simultaneously and the treatment is accelerated.

Particularly advantageously, steps b) and c) can follow one another without an intermediate cleaning operation, for example with water, producing cleaning effluents. Preferably, however, the method comprises, between steps b) and c), an operation of separating the liquid phase of the two-phase mixture resulting from the first of steps b) and c), chronologically, preferably by filtration/sieving and/or decantation and/or flotation and/or cycloning and/or centrifugation and/or drying/evaporation.

In a preferred embodiment, the method further comprises the following step:
d) after the last of steps b) and c), preferably after step c), treatment of the particulate mixture with an acid solution, of pH less than 6, preferably less than 5, more preferably less than 4, more preferably less than 3 or “acid attack”.

The acid attack is intended for the extraction, from the particulate mixture, of silica and/or metals possibly present, in particular compounds of antimony, arsenic, cadmium, cobalt, hexavalent chromium, copper, tin, manganese, mercury, lead, nickel, selenium, tellurium, thallium, vanadium and zinc, to which aluminium in metallic form may also be added.

Advantageously, step d) does not produce any release of hydrocyanic acid, including in the presence of thiocyanates, which is particularly advantageous.

Without being bound by this theory, the inventors explain the results obtained by a stabilizing effect of the basic solution on thiocyanates which would result from a reaction between the alkaline cyanide compound and the thiosulfate compound.

Step d) may be performed after steps b) and c), performed in any order, and before step e).

Step d) follows step b) or c), carried out successively or simultaneously. Preferably, the process does not include an intermediate cleaning operation between step d) and the step preceding it, which avoids the production of cleaning effluents to be treated.

The basic solution may include an alkali and/or alkaline earth hydroxide.

According to the invention, the liquid phase at the end of the step preceding step d) may or may not be separated from the particulate mixture. Preferably, the method comprises, between step d) and the step preceding it, an operation of separating the liquid phase from the two-phase mixture resulting from said preceding step, preferably by filtration/sieving and/or decantation and/or flotation and/or cycloning and/or centrifugation and/or drying/evaporation.

If the used refractory product contained fluorine before step a), step d) advantageously does not produce any release of hydrofluoric acid, including in the presence of thiocyanates, which is particularly advantageous, hydrofluoric acid being very dangerous.

Advantageously, the particulate mixture from step d), i.e. the used refractory product after it has been treated according to the invention, can be used to manufacture a new refractory product without further operations. Advantageously, the new refractory product behaves like a product manufactured from non-recycled raw materials. They have also shown that reuse is possible whether the refractory product is extracted from a metallurgical furnace or an electrolysis cell in a molten salt bath, in particular an electrolysis cell for the production of a non-ferrous metal, in particular aluminum. Finally, the process is simple and does not present a health risk, in particular because it does not involve any complex operation and can be implemented at room temperature and atmospheric pressure.

Step e) may follow step b), preferably follow a step c) subsequent to step b) and, more preferably, follow a step d) subsequent to step c). Preferably, the process does not include an intermediate cleaning operation between these different steps. Preferably, at least after the last of steps b), c) and d), the process however includes an operation of separation of the liquid phase, preferably by filtration/sieving and/or decantation and/or flotation and/or cycloning and/or centrifugation and/or drying/evaporation.

A method according to the invention may further comprise one or more of the following optional and preferred features:
- in step a), a deposit of a slag or a solidified molten bath adheres to the used refractory product extracted from the thermal installation, and, in step a), said deposit is separated, preferably by mechanical action, so as to purify the extracted used refractory product;
- before step b), the thiosulfate solution is prepared by diluting in a solvent, preferably water, a salt of said thiosulfate compound in which the thiosulfate function is linked to at least one atom of a chemical element chosen from metals, in particular zinc, gold, iron, silver, alkalis and alkaline earths, preferably chosen from Na, Ca, K, Mg, K and Ba, or is linked to a cation, preferably ammonium;
- in step b),
- the particulate mixture is soaked in said thiosulfate solution, or
- said thiosulfate solution is sprayed onto the particulate mixture, preferably by mixing the particulate mixture to coat the external surface of substantially all of the particles of the particulate mixture;
- the thiosulfate solution contains more than 5 grams of thiosulfate compound per kilogram of used refractory product;
- the thiosulfate solution provides more than one mole of thiosulfate ions per mole of free cyanide present in said used refractory product;
- steps b) and c) are carried out simultaneously;
- in step c), the basic solution comprises an alkali and/or alkaline-earth oxide, preferably a hydroxide of at least one alkali or alkaline-earth element, and/or a halide of at least one alkali or alkaline-earth element, in particular a chloride of at least one alkali or alkaline-earth element;
- the basic solution preferably comprises a compound chosen from NaOH, KOH, Ca(OH) ₂ , Mg(OH) ₂
- in step d), the acidic solution is preferably sulfuric acid;
- in step e), the new refractory product has the same composition as the used product before use;
- at least one, preferably each step is carried out at a temperature between 15°C and 30°C, preferably at room temperature (around 20 to 25°C).

The invention also relates to the particulate mixture obtained at the end of step b), preferably c), preferably d). This particulate mixture is distinguished from a raw material by the traces of the compounds of the reactions carried out during the treatment.

The invention finally relates to a thermal installation, preferably a metallurgical furnace or an electrolysis cell in a molten salt bath, in particular an electrolysis cell for the production of a non-ferrous metal, in particular aluminum, comprising a particulate mixture resulting from a process according to the invention.

Definitions

• A material with a melting temperature greater than 1500°C is called "refractory". This definition is commonly used by those skilled in the art and is cited in "Refractory materials and technical ceramics (elements of ceramics and technology)", G. Aliprandi, Septima Paris editions, 1979. This work also gives examples of refractory materials, in particular oxides, carbides and nitrides, on pages 297 to 301. A refractory product is a product made of a refractory material.
• A silico-aluminous product is a product made of a material consisting, for more than 50% by mass, of the element silicon and the element aluminum, expressed in oxide form, respectively SiO ₂ and Al ₂ O ₃ , said elements being in the form of the same oxide or different oxides. Conventionally, a silico-aluminous product is manufactured from clay and/or silica and/or alumina. In a silico-aluminous product, the aluminum content expressed in the form Al ₂ O ₃ is less than 70% and the silicon content expressed in the form SiO ₂ is greater than 10%.
• A nitrogen matrix refractory product is a product comprising ceramic grains, for example SiC, bound by a matrix, said matrix comprising or consisting of a SiALON phase. The term "SiAlON" or "silicon oxynitride" means a nitrogenous phase, preferably crystallized, comprising at least the elements Si and N. A SiAlON phase is defined by the formula Si _x Al _y O _u N _v , in which x is greater than or equal to 0, greater than 0.05, greater than 0.1 or greater than 0.2, and less than or equal to 1, less than or equal to 0.8 or less than or equal to 0.4, y is greater than 0, or greater than 0.1, greater than 0.3 or greater than 0.5, and less than or equal to 1, u is greater than or equal to 0, greater than 0.1 or greater than 0.2, and less than or equal to 1 or less than or equal to 0.7, and v is greater than 0, greater than 0.1, greater than 0.2 or greater than 0.5, or greater than 0.7, and less than or equal to 1, at least one stoichiometric indices x, y, u and v being equal to 1. This family includes for example the phases Si ₃ N ₄ , Si ₂ ON ₂ , AlN15R, β'-SiAlON. The AlN15R phase is defined by the formula Si _x' Al _y' O _u' N _v' , in which the stoichiometric indices, normalized with respect to the highest index, are such that 0.12 ≤ x' ≤ 0.33 and 0.78 ≤ y' ≤ 1 and 0.33 ≤ u' ≤ 0.55 and 0.80 ≤ v' ≤ 1. The β'SiAlON phase is defined by the formula Si _x'' Al _y'' O _u'' N _v'' , in which the stoichiometric indices, normalized with respect to the highest index, are such that 0.43 ≤ x'' ≤ 0.75 and 0 ≤ y'' ≤ 1 and 0 <u'' ≤ 1 and 0.9 ≤ v'' ≤ 1.
• A carbon product is a product made of a material containing at least 50% by mass of carbon (in different forms, more or less crystallized or not), in particular anthracite or graphite.
• Concrete is a refractory mixture capable of setting by pouring and possibly with vibration if the mixture is not self-flowing. In hydraulic concrete, setting is obtained by adding water (mass content generally greater than 3%).
• Cement differs from concrete in its grain size and installation method because it is intended to create joints and/or fill gaps with a depth or thickness typically less than 3 cm.
• Rammed earth is a refractory mixture implemented “dry” or with little water (< 3% by mass) by tamping or beading. Hardening is obtained by an organic and/or mineral agent.
• A grout differs from rammed earth by its grain size and its installation method because it is intended to cover surfaces and/or fill gaps with a depth typically less than 3 cm.
• A sintered product is a product obtained by sintering a granular agglomerate, in particular by heat treatment at a temperature below its melting temperature.
• A molten product is a product obtained by melting a mixture of suitable raw materials in an electric arc furnace or by any other suitable technique. The molten material is then poured into a mold, and the resulting product undergoes a controlled cooling cycle to be brought to room temperature without fracturing. Cooling may be free or controlled, depending in particular on the molding technology used.
• By "alkali cyanide compound" is meant any molecule of formula MCN, M being an alkali element and CN being the cyano group −C≡N.
• “Fluorinated compound” means a compound containing fluorine.
• By "thiosulfate compound" is meant any molecule comprising the thiosulfate group, that is to say a chemical group in which, in relation to the sulfate, an oxygen atom has been replaced by a sulfur atom.
• By “acidic compound” and “basic compound” we classically mean compounds whose presence tends to lower and increase the pH of an environment, respectively.
• A block is a monolithic piece with a thickness greater than 50 mm, a width and/or length greater than 300 mm.
• By “two-phase mixture” is meant the refractory product, optionally in the form of a particulate mixture, during treatment and the liquid phase in contact with the refractory product, namely, depending on the embodiment, the thiosulfate solution, the basic solution, or the acid solution. The liquid phase may be in a very small quantity, for example in the case of spraying.
• “Cleaning” means an operation in which substantially all of the liquid phase is removed from a two-phase mixture, so as to recover the solid phase, in particular a particulate mixture, without changing the composition of the solid phase. Cleaning involves a cleaning liquid, for example water, different from said liquid phase and compatible with the following treatment step. Conventionally, said liquid phase is first partially separated from the solid phase, for example by filtration/sieving and/or decantation and/or flotation and/or cycloning and/or centrifugation and/or drying/evaporation, and then the residues of said liquid phase are discharged with the cleaning liquid, in the form of a “cleaning effluent”.

• On distingue un « nettoyage » d’une « séparation », qui ne produit pas d’effluent de nettoyage. Une séparation peut être en particulier réalisée par filtration/tamisage et/ou décantation et/ou flottation et/ou cyclonage et/ou centrifugation et/ou séchage/évaporation. La phase liquide séparée à ultérieurement traiter correspond à la phase liquide du mélange biphasique et n’est avantageusement pas augmentée par l’ajout d’un liquide de nettoyage.

Cleaning takes time. In addition, it is necessary to treat the cleaning effluent. Effluent treatment is expensive and consumes energy.

• A distinction is made between "cleaning" and "separation", which does not produce a cleaning effluent. A separation can in particular be carried out by filtration/sieving and/or decantation and/or flotation and/or cycloning and/or centrifugation and/or drying/evaporation. The separated liquid phase to be subsequently treated corresponds to the liquid phase of the two-phase mixture and is advantageously not increased by the addition of a cleaning liquid.

• Dans un souci de clarté, on utilise les formules chimiques des oxydes ou des non-oxydes pour désigner les teneurs de ces oxydes ou des non oxydes dans une composition. Par exemple, « ZrO₂», « SiO₂» ou « Al₂O₃» désignent les teneurs de ces oxydes et « zircone », « silice » et « alumine » sont utilisés pour désigner des phases de ces oxydes constituées de ZrO₂, SiO₂et Al₂O₃, respectivement.
• Sauf indication contraire, tous les pourcentages relatifs à la composition du mélange particulaire ou du produit réfractaire sont en masse, sur la base du mélange particulaire ou du produit réfractaire, respectivement.
• Une teneur massique d’un oxyde d’un élément métallique se rapporte à la teneur totale de cet élément exprimée sous la forme de l'oxyde le plus stable, selon la convention habituelle de l'industrie.

According to the invention, the liquid phase of the two-phase mixture resulting from step b) or step c) may optionally be separated from the particulate mixture by a simple separation operation, and if it is separated, the particulate mixture may immediately undergo the following treatment step, without producing cleaning effluents, even if the separation is imperfect, and in particular even if residues of the liquid phase remain on the particulate mixture.

• For the sake of clarity, the chemical formulas of the oxides or non-oxides are used to denote the contents of these oxides or non-oxides in a composition. For example, "ZrO ₂ ,""SiO ₂ ," or "Al ₂ O ₃ " denote the contents of these oxides and "zirconia,""silica," and "alumina" are used to denote phases of these oxides consisting of ZrO ₂ , SiO _{2 ,} and Al ₂ O ₃ , respectively.
• Unless otherwise stated, all percentages relating to the composition of the particulate mixture or refractory product are by mass, based on the particulate mixture or refractory product, respectively.
• A mass content of an oxide of a metallic element refers to the total content of that element expressed in the form of the most stable oxide, according to the usual industry convention.

Detailed description Refractory product

A method according to the invention is applicable to any type of used refractory product used in a thermal installation, in particular used in a furnace, preferably a metallurgical furnace or an electrolysis cell in a bath of molten salt(s), in particular an electrolysis cell for the production of a non-ferrous metal, in particular aluminum.

The refractory product may in particular be a molten product or a sintered product (in particular a nitrogen matrix product and/or a SiC product). It may be a carbon product or a silico-aluminous product. It may be a monolithic product obtained by the installation of an unshaped product (in particular a concrete, rammed earth, cement or refractory grout). Preferably, the refractory product is a carbon product, a nitrogen matrix refractory product, a SiC product or a silico-aluminous product.

In the thermal installation, the used refractory product is in the form of a block. Preferably the maximum thickness of the block is less than 500 mm, preferably less than 300 mm, preferably less than 200 mm, preferably less than 100 mm. In a furnace, the used refractory product may be in particular a block of a side wall of the furnace enclosure, a hearth block, a vault block or an insulating refractory part shaped on site before or during assembly of the furnace. In an electrolysis cell, the used refractory product may be in particular a block of a side wall of a tank, a cathode block, or an insulating part, in particular under the cathode block(s).

The refractory product may be a product, preferably sintered, comprising carbon, in particular in the form of carbide, and/or nitrogen, in particular in the form of a nitride matrix binding refractory grains, in particular a matrix of silicon and/or aluminium nitride or oxynitride. Such a product is conventionally used as a lining for metallurgical furnaces. It has good thermal properties and, in the case of carbides, very good resistance to corrosion by molten metals or their slag. In the presence of alkalis and in certain temperature ranges, however, it easily forms alkaline cyanide compounds.

The refractory product may in particular contain more than 20% of carbon dosed in elemental form, in particular more than 70% of SiC. It may contain more than 80%, or even more than 90% or even more than 95% of Carbon C element, in particular for cathode or anodic blocks.

The refractory product may be a silico-aluminous product. Such a product, conventionally used as a coating, may be contaminated by alkaline cyanide compounds, particularly when used in contact with alkalis at high temperatures. It may also be contaminated by migration of alkaline cyanide compounds, for example when used as an insulator, on the back of carbonaceous or nitrogen matrix products or in the lower part of furnaces, under the sole, or under the cathodes of aluminium electrolysis tanks.

By "contamination" we traditionally mean the presence of a compound that is potentially dangerous to humans or their environment. The mass content from which a compound is contaminant varies depending on the compound and its form. In particular, fluorine is very dangerous in the form of HF while it presents much more limited risks in the form of CaF ₂ which is a very stable form. Typically, in refractory products, the following compounds are considered contaminants from the following mass contents:
- cyanide (CN): a content greater than 0.0025%, or 25 ppm;
- free silica: generally above 1%;
- fluorine: above 0.05% when in free form. The mass content of AlF ₃ or even NaF is considered contaminant above 0.5%. The total fluorine content above 1% is considered contamination;
- heavy metals: this depends on the type of compound considered but typically a content above 500 ppm is considered contamination;
- aluminium in metallic form above 0.5% due to reactivity with water and for uncontrolled use, in particular storage in the open air;
- Iron in metallic form above 1%, preferably above 0.5% depending on the intended reuse of the refractory product, in particular depending on whether the treated refractory product is used to manufacture an oxide or non-oxide refractory product.

In step a) , the used refractory product is extracted from the thermal installation in which it was used and contaminated.

Preferably, before extraction, at least a portion of the surface of the used refractory product is covered with a first thiosulfate solution, preferably by means of a roller or by spraying. When the used refractory product belongs to a tank, preferably at least its surface exposed inside the tank is covered with the first thiosulfate solution. Advantageously, the health risk during the extraction operation is reduced.

Thiosulfate solution

The first thiosulfate solution contains a thiosulfate compound, which has the advantage of not generating any particular health risk.

The thiosulfate compound may in particular be provided in the form of a salt in which the thiosulfate function is linked by at least one atom of a chemical element chosen from metals, in particular zinc (Zn), gold (Au), iron (Fe) and silver (Ag), alkalis or alkaline earths, preferably from Na, Ca, K, Mg, K and Ba. In a preferred embodiment, the thiosulfate compound is a sodium or calcium thiosulfate. Such a thiosulfate compound reacts with unstabilized fluorinated compounds or free fluorine by stabilizing them in the form of CaF ₂ or NaF. It is in particular well suited for the reuse of used refractory products from electrolysis cells in a molten salt bath, in particular halides, more particularly fluorinated salts.

The thiosulfate function can be linked to cations such as ammonium, as in the case of ammonium thiosulfate. Advantageously, alkaline earth thiosulfate can react with sodium and aluminum fluorides in the form of alkaline earth fluorides which are generally more stable.

Preferably, a solvent is mixed with a salt, for example in the form of thiosulfate of an alkali or alkaline earth metal or of a cation such as ammonium, so as to form a thiosulfate solution.

Dissolution is preferably carried out at room temperature, typically between 10°C and 30°C.

The solvent may be water, preferably deionized water. A person skilled in the art knows how to adapt the concentration of thiosulfate according to the content of alkaline cyanide compound in the particulate mixture to be treated.

The volume concentration of thiosulfate compound in the solution is preferably greater than 0.5%, preferably greater than 1%, preferably greater than 5%, preferably greater than or equal to 10% and/or preferably less than 25%, preferably less than 20%. This concentration can be evaluated by the amount of thiosulfate compound added during the preparation of the solution. The concentration of thiosulfate compound in the solution is preferably adjusted according to the amount of alkali cyanide compound in the used refractory product.

Preferably, the thiosulfate solution is adjusted so as to contain more than 5 g, preferably more than 10 g, preferably more than 20 g, preferably more than 50 g, and/or less than 100 g of thiosulfate compound per kg of used refractory product .

Preferably, the thiosulfate solution is adjusted to contain more than one mole of thiosulfate ions, preferably more than 1.2 moles, preferably more than 1.5 moles, preferably more than 2 moles of thiosulfate ions per one mole of total cyanide present in said particulate mixture.

In order to improve the preservation, rheology, adhesion and effectiveness of the solution, conventional additives, for example a deflocculant and/or a dispersant and/or a surfactant and/or a biocide, may be added to the thiosulfate solution. In a preferred embodiment, an additive is added which changes color upon formation of thiocyanate, and thus serves as an indicator of the progress of cyanide neutralization.

Preferably, the temperature when applying the first thiosulfate solution is less than 40°C, preferably less than 30°C, preferably less than 25°C. If the refractory product is still hot, typically at a temperature of 40°C or more, it is preferable to allow it to cool before contacting with the first thiosulfate solution. In a preferred embodiment, the temperature of the refractory product is between 5°C and 30°C, preferably between 10°C and 30°C when applying the first thiosulfate solution.

The duration of keeping the refractory product in contact with the first thiosulfate solution is preferably greater than 5 hours, preferably greater than 10 hours, preferably greater than 24 hours, preferably greater than 48 hours and/or less than 100 hours. It is set so as to transform the alkaline cyanide compounds into alkaline thiocyanates, which can be verified by a simple check. The duration can be determined beforehand by a chemical check, in particular by measuring the total cyanide content.

The pressure when applying the first thiosulfate solution is preferably atmospheric pressure.

The extraction of the used refractory product, for example by percussion, for example using a jackhammer or an equivalent tool, is conventionally carried out during a dismantling operation to replace, at least partially, the refractory lining of said thermal installation. During its extraction from the thermal installation, the refractory product may fragment.

After extraction, the used refractory product is preferably freed, preferably by mechanical action, from the deposit of slag or solidified bath which may persist after dismantling of the installation.

The removal of the slag or solidified bath deposit facilitates the evaluation of the quantity of alkaline cyanide compound. It also makes it possible to increase the contact surface of the refractory product with the thiosulfate solution.

Preferably, the pieces of the used refractory product are coated, at least partially with a second thiosulfate solution. Preferably, the second thiosulfate solution has one or more of the necessary or optional characteristics of the first thiosulfate solution, preferably identical to the first thiosulfate solution. The second thiosulfate solution is preferably sprayed onto the pieces.

The conditions for applying the second thiosulfate solution (temperature, pressure, contact time) may be identical to those described previously for the application of the first thiosulfate solution.

The application of the first and/or second thiosulfate solution helps to minimise the risk of contamination through skin contact. Furthermore, if the pieces are stored outside the thermal installation, it reduces the risk of environmental pollution due to leaching by rainwater.

The pieces are typically in the form of blocks, i.e. a particle size that is generally unsuitable for reuse. They therefore undergo particle size reduction so as to obtain a refractory product in the form of a particulate mixture. Preferably, the particle size reduction comprises a crushing operation and/or a grinding operation, dry or wet. Preferably, the particle size reduction comprises crushing followed by grinding.

Preferably, the particle size reduction is such that all dimensions of each particle of the particulate mixture are less than 10 mm, preferably less than 5 mm.

The used and extracted refractory product can be treated in an on-site treatment unit, for example a mobile treatment unit, or, before or after the particle size reduction, be transported, for example in a container, to a treatment unit remote from the thermal installation.

If step b) cannot be carried out immediately after step a), the extracted used refractory product, possibly fragmented into pieces or in the form of a particulate mixture, is stored, preferably in conditions avoiding any contact with the external environment, in particular to protect it from rain.

In step b) , the particles of the particulate mixture are covered, at least partially, preferably substantially completely, with a third thiosulfate solution.

Preferably, the third thiosulfate solution has one or more of the necessary or optional characteristics of the first thiosulfate solution. It is preferably identical to the first thiosulfate solution and/or the second thiosulfate solution.

In general, any deposition or impregnation technique may be suitable for the application of the third thiosulfate solution. The application may in particular be carried out by dipping in the third thiosulfate solution, or by spraying the third thiosulfate solution. Preferably, the third thiosulfate solution is sprayed onto the particles during the particle size reduction operation of step a).

The application of the third thiosulfate solution can be carried out during and/or after the particle size reduction of step a). It is preferable to carry it out during the particle size reduction, because it then limits the risk of emission of dangerous cyanide gases, but also the emission of dust.

The conditions for applying the third thiosulfate solution (temperature, pressure, contact time) may be identical to those described previously for the application of the first thiosulfate solution.

Preferably, the method comprises a separation operation to extract the reaction products with the thiosulfate solution and the residual thiosulfate solution. A simple separation, without the addition of a cleaning liquid, is sufficient. The separation operation is however optional.

Preferably, until the end of step b), the method does not include any steps other than those described above.

In step c), the particulate mixture from step b) is brought into contact with a basic solution.

Basic solution

Preferably, the basic solution comprises a basic compound chosen from an alkali and/or alkaline-earth oxide, preferably a hydroxide of at least one alkali or alkaline-earth element, and/or a halide of at least one alkali or alkaline-earth element, in particular a chloride of at least one alkali or alkaline-earth element.

The basic solution comprises a basic compound preferably chosen from NaOH, KOH, Ca(OH) ₂ , Mg(OH) ₂ . Preferably, the basic solution comprises or is soda and/or lime.

The nature and quantity of the basic solution are determined so that the particles of the particulate mixture are in contact with a liquid phase (i.e. the basic solution) having a pH greater than 7.5 and preferably less than 13.

The concentration of basic compound in the basic solution is preferably between 0.5 and 25%, preferably between 1% and 20% by volume. The balance is preferably water when step b) comprises a separation operation to extract the reaction products of step a) and the residual thiosulfate solution. The concentration of basic compound in the basic solution is preferably adjusted according to the amount of halogen, and in particular fluorine, and silica in the particulate mixture. Preferably, the particulate mixture is washed, preferably with water, so as to reduce the residual mass content of fluorine to less than 1%, stable species such as CaF ₂ being preferred among the acceptable residual species. Preferably, the particulate mixture after this treatment has a residual mass content of free silica of less than 1%.

Preferably, the basic solution is adjusted so as to contain more than 5 g, preferably more than 10 g, preferably more than 50 g, and/or less than 100 g of basic compound per kg of used refractory product .

Preferably, the basic solution is adjusted so as to contain more than one mole of OH ^- ions, preferably more than 1.2 moles, preferably more than 1.5 moles, preferably more than 2 moles of OH ^- ions per one mole of fluorine present in said particulate mixture.

Preferably, the basic solution is adjusted so as to contain more than one mole of OH ^- ions, preferably more than 1.2 moles, preferably more than 1.5 moles, preferably more than 2 moles of OH ^- ions per one mole of free Silica present in said particulate mixture.

Generally speaking, any deposition or impregnation technique can be suitable for this contact.

The application of the basic solution can be carried out in particular by soaking the particulate mixture in the basic solution, or by spraying the basic solution onto the particulate mixture.

Preferably, the temperature during application of the basic solution is less than 40°C, preferably less than 30°C, preferably less than 25°C. In a preferred embodiment, the temperature of the particulate mixture is between 10°C and 30°C during application of the basic solution.

The duration of maintaining the particulate mixture in contact with the basic solution is preferably greater than 1 hour, preferably greater than 5 hours, preferably greater than 10 hours, preferably greater than 15 hours and/or less than 100 hours, preferably less than 24 hours. It is set so as to solubilize all of the fluorine contaminating the refractory product. The pressure during application of the basic solution is preferably substantially atmospheric pressure.

Preferably, the method comprises a separation operation to extract the reaction products with the basic solution and the residual basic solution. A simple separation, without the addition of a cleaning liquid, is sufficient. The separation operation is optional.

Preferably, until the end of step c), the method does not comprise any other steps than those described above. In one embodiment, step c) precedes step b).

In step d), optional and preferred, the particulate mixture undergoes an acid attack with an acid solution.

Acid solution

Preferably, the acidic solution comprises an acidic compound chosen from Brönsted acids, preferably those of formulae H ₂ SO ₄ , HCl, HNO ₃ , CH ₃ COOH, H ₃ PO ₄ , H ₂ CO ₃ or optionally a mixture of these acids.

The acid solution is preferably sulfuric acid.

According to a preferred embodiment, washing, preferably with water, is carried out before step d) in order to remove the liquid phase from the particulate mixture. The nature and quantity of the acid solution are determined so that the particles of the particulate mixture are in contact with a liquid phase, up to a pH of less than 5, preferably less than 4.

According to another embodiment, the acid treatment is carried out without prior washing of the particulate mixture treated in step c). The acid treatment of step d) is then carried out until neutralization of the particulate mixture, i.e. until a pH of between 6 and 7.5, preferably between 6.5 and 7.5 .

The concentration of acidic compound in the acidic solution in contact with the particulate mixture is preferably between 0.5 and 25%, preferably between 1% and 20% by volume, the balance preferably being water. The concentration of acidic compound in the acidic solution is preferably adjusted according to the amount of silica and/or metals, in particular heavy metals, in the particulate mixture.

Preferably, the acid solution is adjusted so as to contain more than 5 g, preferably more than 10 g, preferably more than 50 g, and/or less than 100 g of acid compound per kg of particulate mixture .

Preferably, the acid solution is adjusted so as to contain more than 0.1 mole, preferably more than 0.5 mole, preferably more than 1 mole of H ⁺ ions per mole of metals, in particular chosen from heavy metals, metallic iron and metallic aluminum potentially present in said particulate mixture.

Generally speaking, any deposition or impregnation technique can be suitable for this contact.

The application of the acid solution can be carried out in particular by dipping the particulate mixture in the acid solution, or by spraying the acid solution onto the particulate mixture.

Preferably, the temperature during application of the acid solution is less than 40°C, preferably less than 30°C, preferably less than 25°C. In a preferred embodiment, the temperature of the particulate mixture is between 10°C and 30°C during application of the acid solution.

The duration of keeping the particulate mixture in contact with the acid solution is preferably greater than 5 hours, preferably greater than 10 hours, preferably greater than 24 hours, preferably greater than 48 hours and/or less than 100 hours.

The pressure when applying the acid solution is preferably approximately atmospheric pressure.

Preferably, the method comprises a separation operation to extract the reaction products from the particulate mixture and the acid solution. A simple separation, without the addition of a cleaning liquid, is sufficient. The separation operation is optional.

Preferably, until the end of step d), the method does not include any steps other than those described above.

Tests have shown that step d) makes it possible to extract silica and/or metals, particularly heavy metals, even in the presence of the reaction products from the previous steps.

Preferably, the particulate mixture is treated so as to reduce the mass content of crystalline silica to less than 1% and/or the mass content of heavy metals to less than 0.1% and/or less than 1% for the elements Fe and/or Al.

In step e),the particulate mixture is used to manufacture a new refractory product.

The particle size distribution can be adapted for this purpose, for example by additional crushing and/or grinding.

Step e) preferably comprises the following steps:
A) preparation of a starting charge comprising a particulate mixture from steps b) and c) and, preferably, from step d), and a solvent, preferably water;
B) shaping said starting charge so as to form a preform;
C) optionally, sintering said preform so as to obtain a sintered product.

These steps are known to the person skilled in the art who knows how to adapt them to the desired sintered product. In particular, he knows how to adapt the solvent content, in particular according to the particle size and chemistry of the particulate mixture. He can also add shaping additives to the starting charge intended to facilitate shaping.

Examples

In order to evaluate the ability of a treated refractory product to release cyanide compounds, the following leaching test was implemented.

A used SiC nitride block was crushed and then sieved to obtain seven identical samples of a particulate mixture, the largest particle dimension being less than 2 mm.

The composition of the block was as follows:

Chemical analysis, % by mass SiC 56 If ₃ N ₄ 20 O (oxygen element) 2 Na total 8 C free 5 F (total) 5 Al (total) 1 Fe (total) 1 Al in metal form 0.5 Total heavy metals
(Sb, As, Cr, Cu, Sn, Mn, Hg, Pb, Ni, Se, Te, Tl, V and Zn) 1

The SiC and Si ₃ N ₄ contents were determined from the elemental contents of bound carbon (by difference between total carbon and free carbon) and nitrogen respectively, measured by LECO analyzer in accordance with the protocol defined by the ANSI B74.15-1992-(R2007) standard. The oxygen element content was measured, after fusion under inert gas, using an analyzer marketed under the reference TC-436 supplied by the company LECO Corporation. The Al and Na contents were measured by X-ray fluorescence spectroscopy. The Fluorine content was measured by ion chromatography preceded by extraction by pyrohydrolysis.

Each 10 gram sample was soaked for 48 h, at a temperature of 20°C and under atmospheric pressure, in a volume of a “soaking solution”, without stirring:
- the first sample was soaked more precisely in 200 ml of water (example 1);
- the second sample was soaked in a solution of 200 ml of water containing 1 mole per liter of sodium sulfate (example 2);
- the third sample was soaked in a solution of 200 ml of water containing 1 mole per liter of sodium thiosulfate, i.e. a volume concentration of sodium thiosulfate of 2% and a mass ratio of sodium thiosulfate compound to SiC of approximately 0.5 (example 3);
- the fourth sample was soaked in a solution of 200 ml of water containing 1 mole per liter of calcium thiosulfate, or 2% volume concentration of sulfate (example 4);
- the fifth sample was soaked in a solution of 200 ml of water containing 1 mole per liter of sodium thiosulfate, then, after separation by sieving, the particulate mixture was soaked in a solution of 200 ml of water containing 1 mole per liter of NaOH (example 5);
- the sixth sample was soaked in a solution of 200 ml of water containing 1 mole per liter of calcium thiosulfate and 1 mole of NaOH per liter (example 6);
- the seventh sample was soaked in a solution of 200 ml of water containing 1 mole per liter of NaOH per liter (example 7).

The 2% volume concentration of thiosulfate represents an input of two moles of thiosulfate per mole of cyanide present in the used refractory product.

The total CN content of each solution in which a sample was soaked was measured. The analytical method used complies with ISO 14403-2:2012 Water quality — Determination of total cyanide and free cyanide using flow analysis (FIA and CFA) — Part 2: Method using continuous flow analysis (CFA). The results are presented in the following Table 2:

Ex 1* Ex 2* Ex 3 Ex 4 Ex 5 Ex 6 Ex 7* Soaking solution water sodium sulfate sodium thiosulfate calcium thiosulfate sodium thiosulfate then soda calcium thiosulfate and sodium hydroxide soda pH of the soaking solution including the refractory sample 13 8 10 10 14 14 14 pH after 48 hours of soaking 13 7 9 9 14 14 14 Total cyanide content mg/l 450 240 18 7 20 19 350 Limit of quantification mg/l < 1

* Comparative examples

These results show the remarkable effectiveness of the invention in reducing the cyanide level, the maximum threshold targeted being 20 mg/litre of leachate.

A comparison of Examples 3 to 6 according to the invention with comparative Examples 1 and 2 shows that the thiosulfate solution allows very effective neutralization of cyanides. The additional treatment with a basic solution (soda) after or simultaneously with contact with the thiosulfate solution allows sufficient cyanide neutralization activity by the thiosulfate to be maintained.

Comparative example 7 shows, on the other hand, that treatment with a basic solution alone does not allow the total cyanide content to be sufficiently reduced.

Other tests have also shown that soda can reduce contamination by fluorine and free silica.

At the end of the treatment, the particulate mixture treated according to the invention had a composition suitable for its use as a secondary material, including for the manufacture of new refractory products, in particular sintered products, for thermal installations identical to that from which the used refractory product was extracted in step a).

As is now clearly apparent, the invention provides a simple reuse solution, without any particular risk to the environment or operators, and suitable for various refractory products contaminated by an alkaline cyanide compound, and in particular suitable for refractory products containing fluorine. Advantageously, no cleaning operation is required.

Of course, the invention is not limited by the examples, provided for illustrative purposes only.

In particular, if the used refractory product is brought into contact with a thiosulfate solution during and after extraction, the same or different solutions may be used during extraction and after extraction.

Claims (20)

Method for reusing a used refractory product initially placed, in a service position, within a thermal installation and contaminated by an alkaline cyanide compound, said method comprising the following successive steps:
has)
- extraction of the used refractory product out of the service position,
- particle size reduction of the used refractory product so as to obtain a particulate mixture;
b) before said extraction and/or during said extraction, and/or between said extraction and said particle size reduction and/or during said particle size reduction of step a) and/or after step a), contacting the particulate mixture with a solution comprising a thiosulfate compound, or "thiosulfate solution", so as to obtain a two-phase mixture;
e) separation of the liquid phase from the two-phase mixture and use of the particulate mixture to manufacture a new refractory product. Method according to the preceding claim, in which step b) is simultaneous with the particle size reduction of step a). A method according to any preceding claim, comprising the following step:
c) after step a), subsequently, simultaneously or before step b), and before step e), bringing the particulate mixture into contact with a solution with a pH greater than 8, or “basic solution”. Method according to the immediately preceding claim, in which steps b) and c) follow one another without an intermediate cleaning operation. Method according to claim 3, wherein steps b) and c) are carried out simultaneously. A method according to any one of the three immediately preceding claims, in which the basic solution comprises an alkali and/or alkaline-earth oxide and/or a halide of at least one alkali or alkaline-earth element. A method according to the immediately preceding claim, wherein the thiosulfate compound is an alkali metal thiosulfate or an alkaline earth metal thiosulfate. A method according to any one of the four immediately preceding claims, comprising the following step:
d) after steps b) and c), carried out in any order, and before step e), treatment of the particulate mixture with an acid solution, of pH less than 5 or “acid attack”. Method according to the immediately preceding claim, in which step d) follows step b) or c), carried out successively or simultaneously, without an intermediate cleaning operation. A method according to any one of the two immediately preceding claims, in which the basic solution comprises an alkali and/or alkaline earth hydroxide. A method according to any preceding claim, wherein, in step a), a deposit of a slag or a solidified molten bath adheres to the used refractory product extracted from the thermal installation, and wherein, in step a), said deposit is separated, so as to purify the extracted used refractory product. A method according to any preceding claim, wherein the thiosulfate solution comprises more than 5 grams of thiosulfate compound per kilogram of used refractory product. A method according to any preceding claim, wherein the thiosulfate solution provides more than one mole of thiosulfate ions per mole of free cyanide present in said used refractory product. Process according to any one of the preceding claims, in which, before step b), the thiosulfate solution is prepared by diluting in a solvent a salt of said thiosulfate compound in which the thiosulfate function is linked to at least one atom of a chemical element chosen from alkali metals and alkaline earth metals. A method according to any preceding claim, wherein, in step b),
- the particulate mixture is soaked in said thiosulfate solution, or
- said thiosulfate solution is sprayed onto the particulate mixture. A method according to any preceding claim, wherein in step e) the new refractory product has the same composition as the used product before use. Method according to any one of the preceding claims, in which the thermal installation is a metallurgical furnace or an electrolysis cell in a bath of molten salt(s). Particulate mixture obtained at the end of a step c) or d) of a process according to any one of the preceding claims. A method of treating a refractory product contaminated with an alkaline cyanide compound, said method comprising contacting the used refractory product with a solution comprising a thiosulfate compound. A method according to the immediately preceding claim, wherein said solution comprises a catalyst containing a metal, preferably copper, the molar content of catalyst is preferably greater than 5% and less than 50%, as a percentage based on the number of moles of thiosulfate compound.