WO1995029876A1 - Heavy metal cation recovering agent including a silicate or aluminosilicate type compound or a carbonate type compound, and a carrier - Google Patents

Abstract

An agent for recovering heavy metal cations from an aqueous effluent, including either a silicate or aluminosilicate type compound, e.g. an alkali metal silicate or aluminosilicate, or a carbonate type compound, e.g. an alkali metal carbonate, as well as a carrier, preferably a clay. The agent may also form a cation stabiliser. Said agent is useful for removing or stabilising heavy metal cations in water used for cleaning flue gases from waste incineration, particularly household refuse and industrial waste incineration.

Description

 HEAVY METAL CATION CAPTURING AGENT COMPRISING A SILICATE OR ALUMINOSILICATE COMPOUND OR A TYPE COMPOUND

CARBONATE AND A SUPPORT

The present invention relates to an agent for capturing heavy metal cations contained in a medium, in particular an aqueous effluent.

It also relates to an agent for stabilizing said heavy metal cations.

The incineration of waste is subject to a fairly strict regulatory framework. In particular, legislation in the field of discharges of heavy metals into the natural environment is evolving. The water used to wash (or purify) fumes from incineration plants for household waste or industrial waste, in particular industrial effluents such as spent sulfuric acids, are media containing heavy metal cations. Similarly, some soils are polluted by the presence of such cations.

Thus, in the field of water for washing smoke from household waste incineration plants, the best-known process for removing heavy metal cations consists of basic precipitation carried out with lime, assisted by a finish. using a sulfur-based precipitating agent; the subsequent decantation / separation step is generally improved by the incorporation of a flocculating agent. However, this process has a number of drawbacks. In particular, legislation in the field of discharges of heavy metals into the natural environment is evolving. The water used for washing (or purifying) smoke from waste incineration plants, in particular household or industrial waste, industrial effluents such as sulfur effluents, for example used sulfuric acids, are media containing heavy metal cations. Similarly, some soils are polluted by the presence of such cations.

Thus, in the field of water for washing fumes from waste incineration plants, for example household waste, a conventional process for removing heavy metal cations consists of basic precipitation carried out with lime, assisted by '' finishing with a sulfur-based precipitating agent; the subsequent decantation / separation step is generally improved by the incorporation of a flocculating agent.

However, this process has a number of drawbacks. In particular, a very large volume of mud is generated by lime precipitation; this sludge, after filtration and compaction in the form of a cake, must currently be placed in special landfills. In addition, the use of organic materials, such as the sulfur-based precipitating agent and the flocculating agent, which are found at least in part in the cake, adversely affects the landfill (degradation with the formation of reducing gases). over time). Finally, the mud composition obtained can be difficult to stabilize (or immobilize) by current techniques. The presence of a large quantity of calcium in this sludge can strongly inhibit stabilization (or immobilization) operations, such as stabilization by vitrification. However, future regulations relating to the storage of ultimate special waste make it necessary to stabilize (or immobilize) the cake, before admission to the storage site, in order to very significantly reduce the leaching of this type of waste.

The object of the present invention is in particular to provide a means allowing efficient capture (or elimination) of heavy metal cations and not having the above-mentioned drawbacks. To this end, the present invention provides a new agent for capturing (or eliminating) heavy metal cations present in a medium, said agent being a mineral product comprising, on the one hand, either at least one compound of the silicate or aluminosilicate type. , or at least one compound of the carbonate type, and, on the other hand, at least one support, preferably a clay. It also relates to an agent for stabilizing said heavy metal cations comprising said capture agent.

The Applicant has surprisingly found that the use of a heavy metal cation capture agent as defined above, in particular when it contains a compound of the carbonate type, makes it possible to effectively eliminate said cations from the medium containing them. and, advantageously, an improvement in the settling and separation of the sludge obtained / liquid supernatant (in the case where the agent contains a compound of the carbonate type), a certain improvement in the ability to stabilize this sludge, that is to say, a certain improvement in the ability of the latter to resist leaching and a reduction in the residual calcium content of the precipitate compared to what is obtained with the conventional method described above.

Thus, one of the objects of the invention is an agent for the capture (or elimination) of heavy metal cations contained in a medium, characterized in that said agent comprises: - either at least one compound of the silicate or aluminosilicate type, hereinafter called compound A or either at least one compound of the carbonate type, hereinafter called compound B; and

- At least one support (or substrate), which is preferably a clay. By heavy metal cations, is meant in particular the metal cations of valence greater than or equal to 2, preferably equal to 2, and in particular those chosen from antimony, i arsenic, bismuth, cadmium, chromium, cobalt , copper, tin, manganese, molybdenum, nickel, gold, lead, thallium, tungsten, zinc, iron, metals of the actinide family.

The heavy metal cations more particularly targeted by the present invention are cadmium, chromium, copper, nickel, lead, zinc and iron.

Said medium can preferably be liquid.

This medium can thus be constituted by an aqueous effluent, in particular by the water for washing (or purifying) the smoke from incineration of waste, in particular household waste, industrial waste, hospital waste, by washing water. solid matter, such as earth, containing heavy metals, by aqueous surface treatment effluents.

The mineral product for capturing (or eliminating) the heavy metal cations contained in a medium according to the invention, which can be used to purify said medium, can be considered as a composite product formed:

- at least one active principle (or precipitation agent) consisting either of at least one compound A or either of at least one compound B, and

- At least one support (or substrate), preferably consisting of at least one clay.

It should be noted that the active principle of the agent according to the invention consists only of at least one compound A or only of at least one compound B; in other words, the agent according to the invention does not contain both a compound A and a compound B. Compound A is preferably a silicate or an aluminosilicate of an alkali metal, in particular of sodium or potassium .

Advantageously, the compound A is a sodium silicate.

Said sodium silicate then generally has an SiOg / ^ O molar ratio of between 0.5 and 3.8. Compound B is preferably an alkali metal carbonate or a hydroxycarbonate chosen from hydrotalcite and dawsonite.

Hydrotalcite is a basic carbonate of magnesium and aluminum. Dawsonite is a basic aluminum and sodium carbonate.

Advantageously, the compound B is an alkali metal carbonate, preferably sodium.

According to a very preferred embodiment of the invention, the agent according to the invention comprises:

- at least one compound of the carbonate type (compound B); and - at least one support (or substrate), which is preferably at least one clay.

The active principle then only consists of at least one compound B.

It is found that the use of an agent according to this very preferred embodiment of the invention allows, in particular, a more effective elimination of the heavy metal cations contained in a medium compared to that obtained by using an agent according to the invention comprising a compound A.

The agent according to the invention is generally in the form of a powder or a granule.

The support contained in the agent according to the invention is, preferably, a clay.

The clay then contained in the agent according to the invention can be of natural or synthetic origin.

The clay used in the agent according to the invention advantageously has a high content by weight of Al ₂ O ₃ ; this content is for example between 20 and 40%.

It is preferred, according to the invention, to use a clay of lamellar or phyllosilicate structure.

It is thus possible to use a clay chosen from the group comprising kaolinites, serpentines. The clay can also be chosen from the group comprising montmorillonites, bentonites (in particular alkaline), talc, mica.

Preferably, the clay chosen belongs to one of these two groups.

It is optionally possible to use a clay of chlorite type structure.

The agent according to the invention generally contains, as clay, a montmorillonite. The content of support, in particular clay, of the agent according to the invention is usually between 10 and 90% by weight, for example between 20 and 80% by weight or between 10 and 30% by weight, relative to the total weight of said agent.

In general, the agent according to the invention has a water content of between 1 and 20% by weight. This water content is determined by measuring the loss of mass by calcination at 350 ° C for 6 hours.

The agent according to the invention can have a mass average particle size of between 15 and 100 μm, for example between 20 and 75 μm.

The agents according to the invention are prepared by any suitable process.

Thus, according to a first variant, it is possible to introduce a support, in particular a clay, generally with stirring, either in an aqueous solution of compound A or in an aqueous solution of compound B

The solution then obtained is, generally after having been subjected to stirring, dried, in particular by a flash process or, preferably, by atomization. Likewise, according to a second variant, an aqueous solution of compound A or either an aqueous solution of compound B is added, generally with stirring, to a support, in particular a clay, previously introduced into a granulator. 'Perform using, for example, a plate granulator and is usually supplemented by drying, preferably in a fluid bed.

In the case where the compound A is a sodium silicate, an aqueous sodium silicate solution is generally employed having a molar ratio S \ 0_JrAa ₂ 0 of between 0.5 and 3.8 and a silicate concentration, expressed as SiO, between 0.1 and 10 mol / l, for example between 0.2 and 8 mol / In the case where compound B is a sodium carbonate, an aqueous solution of sodium carbonate with a concentration of carbonate, expressed as Na ₂ C0 ₃ , between 0.1 and 5 mol / l, for example between 0.2 and 3 mol / l.

The stirring operation is generally carried out for a period of between 5 and 45 minutes, for example between 10 and 25 minutes. The drying step is usually carried out at a temperature between about 40 and 500 ° C, using any suitable device.

Drying can be done by atomization, in particular in the first variant.

To this end, any suitable type of atomizer can be used, in particular a BUCHI type atomizer, a turbine, nozzle, liquid pressure or two fluid atomizer.

The inlet temperature is generally around 230 to 250 ° C in the case of a BUCHI type atomizer and of the order of 450 ° C in the case of a turbine or nozzle atomizer; similarly, the outlet temperature is usually about 110 to

140 ° C in the case of a BUCHI type atomizer and of the order of 125 ° C in the case of a turbine or nozzle atomizer.

One can also use, for drying, a device such as that described in US Pat. No. 4,970,030, device in which, in general, the inlet temperature is of the order of 500 ° C. and the outlet temperature of the around 150 ° C (flash process).

Drying can be done in a fluid bed, in particular in the second variant. At the end of the preparation, a product is obtained in which the active principle is, advantageously, intimately linked to the support.

The use of at least one agent according to the invention for removing heavy metal cations from a medium containing them, in particular a liquid effluent (or solution), in particular an aqueous effluent (or solution) can be done as follows:

The agent according to the invention is introduced into the liquid effluent to be treated, with stirring. The final pH of the suspension containing said agent is preferably between 7 and 9 or adjusted to a value between 7 and 9 by adding a base or a acid, if said agent comprises a compound of the silicate or aluminosilicate type (compound A), either between 8 and 11 or adjusted to a value between 8 and 11 by addition of a base or an acid, if said agent a compound of the carbonate type (compound A). The final pH depends on the quantity of agent according to the invention introduced into the liquid effluent to be treated and on the initial pH of said effluent. Agitation is continued, for example for 5 to 60 minutes. The suspension is then left to settle (decant) at room temperature for a certain time, generally between 0.5 and 24 hours, in particular between 0.5 and 6 hours. The settling time can be reduced if rapid settling methods known to those skilled in the art are used. Then, the precipitate formed is separated, that is to say the agent according to the invention loaded with heavy metal cations, by decantation, filtration and / or centrifugation of the suspension.

Generally, the medium to be treated, in particular when it consists of a liquid effluent (in particular an aqueous effluent (a solution)), contains 0.5 to 6000 mg / l, for example 1 to 1000 mg / l, in particular 2 to 100 mg / l heavy metal cations. The amount of agent according to the invention added to the medium to be treated is such that the molar ratio (SiO ₂ + CO ₃ * -) / (cations present in the medium to be treated) is, in general, between 0.7 and 2.5, for example between 1.0 and 2.2, in particular between 1.1 and 1.9. The term “cations present in the medium to be treated” is understood here to mean heavy metal cations and Ca ²⁺ cations. It is noted, in particular in the case where an agent comprising a compound of the carbonate type (compound B) is used, that, after separation, the precipitate formed, that is to say the agent according to the invention charged with heavy metal cations, has a relatively satisfactory ability to stabilize (or immobilize). It also exhibits acceptable behavior with respect to leaching: it is in fact relatively little leachable, that is to say that it releases few heavy metal cations which it contains when it is put in presence of water; the quantities of heavy metal cation chemical species in leachate obtained from conventionally carried out leaching tests are relatively small.

Thus, another subject of the invention consists of an agent for stabilizing (or immobilizing) heavy metal cations contained in a medium, characterized in that it comprises at least one agent as described above.

If the presence of a support in the agent according to the invention allows, in general, localized precipitation on the periphery of said support, it also makes it possible, in particular in the case of clay, to favor, surprisingly, in the case where the agent according to the invention comprises a compound of the carbonate type, decantation, in particular to increase the speed of decantation. Likewise, it can also make it possible to reduce the very low content of heavy metal cations in the leachate as mentioned above. The following examples illustrate the invention without, however, limiting its scope.

EXAMPLE 1

To 830 ml of an aqueous solution of sodium silicate with a Siθ ₂ / Na ₂ O molar ratio equal to 1, with a silicate concentration, expressed as Si0 ₂ , of 0.2 mol / l, add, with stirring, 75 grams of montmorillonite clay. Agitation is continued for 15 minutes. The suspension obtained is finally dried using an atomizer of the BUCHI type.

The flow rate is 10 ml / min; the inlet temperature is set at 240 ° C, the outlet temperature at 135 ° C.

The product obtained contains sodium silicate and 69% by weight of montmorillonite clay; it has a water content of 12% by weight.

EXAMPLE 2

75 grams of montmorillonite clay are added to an aqueous solution of sodium carbonate obtained by dissolving 17.67 grams of! Ma ₂ C0 ₃ in 800 ml of water.

Agitation is continued for 15 minutes.

The suspension obtained is finally dried using an atomizer of the BUCHI type. The flow rate is 10 ml / min; the inlet temperature is fixed at 250 ° C, the outlet temperature at 130 ° C.

The product obtained contains sodium carbonate and 78% by weight of montmorillonite clay; it has a water content of 3% by weight.

EXAMPLE 3

300 ml of a 0.5 M NaOH / Na ₂ C0 ₃ 0.5 M solution are introduced into a thermostated double jacket reactor.

Then added, with stirring, 75 grams of montmorillonite clay.

After stirring for 30 minutes and raising the temperature to 80 ° C., 260 ml of MgCl ₂ O, 5 M / 0.165 M AICI ₃ solution are added simultaneously, and 260 ml of 1.3 M NaOH / NaCO ₃ O.O8 M solution, for 45 minutes.

The solution obtained is then matured at 80 ° C for 30 minutes. The precipitate obtained is centrifuged, then washed in 900 ml of water. It is then centrifuged again and finally dried at 50 ° C and demottled. The product obtained contains hydrotalcite and 85% by weight of montmorillonite clay; it has a water content of 1% by weight.

EXAMPLE 4

Is introduced over 5 minutes, with stirring, into 100 ml of a CuCl ₂ solution having a Cu ²⁺ concentration of 50 mg / l, 657 mg of the product prepared in Example 3, amount corresponding to a CO molar ratio ₃ ² 7Cu ²⁺ of approximately 1, 8 and stirring is continued for 30 minutes.

The pH of the suspension obtained, which is 10, is brought to 12 by adding 1.5 ml of 1N sodium hydroxide.

The suspension is then left to stand at room temperature for 4 hours.

Then it is centrifuged at 3000 rpm for 5 minutes.

The supernatant then has a Cu ²⁺ concentration, measured by atomic absorption, of 3 mg / l.

EXAMPLE 5

The procedure is as in Example 4, except that no sodium hydroxide is added (and therefore the pH of the suspension remains at 10).

The supernatant obtained then has a Cu ²⁺ concentration, measured by atomic absorption, of less than 1 mg / l.

EXAMPLE 6

The procedure is as in Example 5, except that 0.5 ml of 0.5 N hydrochloric acid is added with the product prepared in Example 3, the pH of the suspension obtained then being 9.

The supernatant obtained then has a Cu ²⁺ concentration, measured by atomic absorption, of less than 1 mg / l. EXAMPLE 7

The procedure is as in Example 5, except that 2 ml of 0.5N hydrochloric acid are added with the product prepared in Example 3, the pH of the suspension obtained then being 7, 2.

The supernatant obtained then has a Cu ²⁺ concentration, measured by atomic absorption, of less than 1 mg / l.

EXAMPLE 8

To an aqueous solution of sodium carbonate obtained by dissolving 100 grams of Na ₂ C0 ₃ in 1 liter of water, 100 grams of montmorillonite clay are added with stirring.

The volume is brought to 1.2 liters by adding water. Agitation is continued for 15 minutes.

The suspension obtained is finally dried using an atomizer of the BUCHI type.

The flow rate is 7 ml / min; the inlet temperature is set at 240 ° C, the outlet temperature at 118 ° C.

The product obtained contains sodium carbonate and 25% by weight of montmorillonite clay; it has a water content of 2% by weight.

EXAMPLE 9

An aqueous effluent is obtained from the following various salts: CaCl ₂ , 2H ₂ 0; FeCI ₃ , 6H ₂ O; ZnCI ₂ ; PbCI ₂ ; CuCI ₂ ; NiCI ₂ ; 3CdSO ₄ , 8H ₂ O; NaCI; Na ₂ SO ₄ .

This effluent has a pH of 3 and the following contents expressed in mg / ml: Fe ^ + Fe ³ *: 50 Pb ² *: 10 Zn ²⁺ : 150 Ca ²⁺ : 1000

Cu ²⁺ : 5 CI ^* : 40000 Ni ²⁺ : 5 SO ₄ ² ": 1000

CD ²⁺ : 5

The pH of this effluent is adjusted to the value 4 by adding 0.1 N sodium hydroxide. 1.17 grams of the product prepared in Example 8 are introduced, with stirring, into 250 ml of said effluent and stirring is continued. during 30 minutes. The pH of the suspension obtained is 8.5.

The suspension is then left to stand at room temperature for 4 hours.

Then it is filtered on filter paper. The following concentrations of Fe ⁺ , Fe ³ *, Cu ²⁺ , Ni ²⁺ , Cd ²⁺ , Pb ^{2 * 1 *} , Zn ⁺ , Ca ²⁺ of the filtrate are measured, expressed in mg / ml:

Fe ^ + Fe ³ *: <0.2 Cd ²⁺ : <0.2

Zn ²⁺ : <0.5 Pb ²⁺ : <0.5

Cu ²⁺ : <0.2 Ca ²⁺ : 1

Ni ²⁺ : <0.2

EXAMPLE 10

The procedure is as in Example 9, until the end of the 4 hour rest phase.

Then the agitation is restarted for 5 minutes.

The suspension obtained is then poured into a graduated cylinder with a useful capacity of 250 ml and a base surface equal to 12.82 cm ² . From the initial slope of a curve, known as the sedimentation curve, representing the height of the apparent volume of sediment formed over time (decantation) for 120 minutes, a decantation speed equal to 2.5 m / is determined. h.

EXAMPLE 11

Is introduced into 100 ml of a solution of ZnCI ₂ and CaCl ₂ , 2H ₂ O, having a concentration of Zn ²⁺ of 750 mg / ml and of Ca ²⁺ of 1000 mg / ml, of the product prepared in the Example 8 in an amount such that the pH of the solution obtained is equal to 7. After stirring for 15 minutes, the suspension obtained is poured into a graduated cylinder.

The following table indicates the apparent volume of sediment formed at various times during settling:

Time (min) 0 25 50 110 170

Volume (ml) 100 72 25 15 10

Examples 10 and 11 illustrate the good settling behavior of the precipitate obtained using a product according to the invention.

Claims

1 - Agent for capturing heavy metal cations contained in a medium, characterized in that said agent comprises: - either at least one compound A of the silicate or aluminosilicate type, or at least one compound B of the carbonate type; and - at least one support. 2 - Agent according to claim 1, characterized in that it comprises: - at least one compound A of the silicate or aluminosilicate type; and - at least one support. 3 - Agent according to claim 2, characterized in that said compound A is a silicate or an aluminosilicate of alkali metal. 4 - Agent according to claim 3, characterized in that said compound A is a sodium or potassium silicate or aluminosilicate. 5 - Agent according to claim 4, characterized in that said compound A is a sodium silicate. 6 - Agent according to claim 5, characterized in that said sodium silicate has a molar ratio S \ 0_JNa ₂ 0 between 0.5 and 3.8. 7 - Agent according to claim 1, characterized in that it comprises: - at least one compound B of the carbonate type; and - at least one support. 8 - Agent according to claim 7, characterized in that said compound B is an alkali metal carbonate or a hydroxycarbonate chosen from hydrotalcite and dawsonite. 9 - Agent according to claim 8, characterized in that said compound B is an alkali metal carbonate. 10 - Agent according to claim 9, characterized in that said compound B is a sodium carbonate. 11 - Agent according to one of claims 1 to 10, characterized in that it has a support content of between 10 and 90% by weight. 12 - Agent according to one of claims 1 to 11, characterized in that said support is a clay. 13 - Agent according to claim 12, characterized in that said clay is a clay of natural or synthetic origin. 14 - Agent according to one of claims 12 and 13, characterized in that said clay is a montmorillonite. 15 - Agent according to one of claims 1 to 14, characterized in that it has an average particle size by mass of between 15 and 100 microns. 16 - Agent for stabilizing heavy metal cations contained in a medium, characterized in that it comprises at least one agent according to one of claims 1 to 15. 17 - Agent according to one of claims 1 to 16, characterized in that said medium is an aqueous effluent. 18 - Agent according to claim 17, characterized in that said aqueous effluent is constituted by the washing waters of the waste incineration fumes.