WO1997005068A1

WO1997005068A1 - Method for removing heavy metals from liquid effluents

Info

Publication number: WO1997005068A1
Application number: PCT/FR1996/001150
Authority: WO
Inventors: Isabelle Baudin; Christophe Anselme
Original assignee: Lyonnaise Des Eaux
Priority date: 1995-07-28
Filing date: 1996-07-22
Publication date: 1997-02-13
Also published as: FR2731422A1; FR2731422B1

Abstract

A method for removing heavy metals from liquid effluents, particularly water used for scrubbing industrial waste or household refuse incinerator fumes, wherein the liquid effluent containing the heavy metals to be removed is pre-neutralised, particularly using soda, to raise the pH of the liquid effluent to around 1-4 and thereby precipitate out the metal hydroxides; the resulting pre-neutralised liquid effluent is contacted with a metered amount of a final neutralisation additive to adjust the pH of the effluent to around 9, wherein said additive binds the heavy metals; the heavy metals are separated; and the concentrated solid thus separated is filtered to give a liquid material and a heavy metal-containing solid material that may then be vitrified.

Description

Method for removing heavy metals from liquid effluents

The present invention relates to a process intended to ensure the elimination of heavy metals in liquid effluents, in particular in water for washing fumes from incinerators of household or industrial waste, without these fields of application being can be considered as limiting.

We know the difficulty that we encounter at present in the elimination of heavy metals contained in liquid effluents. However, this elimination is essential in order to comply with the standards currently in force relating to liquid discharges into the natural environment. These difficulties are due in particular to the highly acidic pH of these effluents which requires prior neutralization, to the high concentrations of heavy metals (of the order of 200 mg / l) to the various suspended solids requiring decantation and extensive filtration resulting in large volumes of sludge to be treated before they can be landfilled.

The present invention has set itself the objective of providing a process for removing heavy metals contained in liquid effluents, and in particular in washing water from waste incinerators which can be used industrially, in particular providing the following advantage: minimization of solid discharges, the method according to the invention also rendering these solid discharges inert;

Consequently, the method according to the present invention is characterized in that it consists in:

- subject the liquid effluent containing the heavy metals to be eliminated to a pre-neutralization, for example using sodium hydroxide, so as to raise the pH of the liquid effluent by a value of 1 to 4, and to precipitate thus metal hydroxides;

subjecting the liquid effluent thus pre-neutralized to the metered action of a final neutralization additive making it possible to bring the pH of the effluent to be treated to a value of the order of 9, this additive ensuring the fixing of heavy metals;

- separate heavy metals and;

- Filter the concentrated solid from the separation step to obtain on the one hand the solid materials containing heavy metals which can then be subjected to vitrification, and on the other hand the liquid material. As specified above, the pre-neutralization step can be carried out by adding NaOH to the liquid effluent. The final neutralization step is carried out by adding, in the liquid effluent whose pH was raised to approximately 4 during the pre-neutralization, an additive capable of removing heavy metals in an acid medium. This final neutralization additive may for example be a zeolite, a hydroxycarbonate, an ion exchange material, composite mixtures of clays + silicates + carbonates. The invention gives particularly satisfactory results using the heavy metal cation capture agents as described and claimed in two French patent applications filed on the same day as this application, by the company RHONE-POULENC CHIMIE and entitled:

"Heavy metal cation capture agent comprising a compound of the silicate or aluminosilicate type, and a compound of the carbonate type". and

"Heavy metal cation capture agent comprising a compound of the silicate or aluminosilicate type or a compound of the carbonate type and a support".

The agent described and claimed in one of these two applications comprises at least one compound of the silicate or carbonate type, which is preferably a silicate or an aluminosilicate of an alkali metal, in particular of sodium or potassium, and at least one compound of the carbonate type, which is preferably an alkali metal carbonate, in particular of sodium, or a hydroxycarbonate chosen from hydrotalcite and dawsonite. The present agent, generally a molar ratio of CO ₃ ² ₂ ./SIO between 0.05 and 10. Preferably, this agent further comprises at least one support, in particular a clay.

The agent described and claimed in the other application comprises at least one support, in particular a clay, and either at least one compound of the silicate or aluminosilicate type, in particular a silicate or an aluminosilicate of an alkali metal, for example sodium or potassium, preferably at least one compound of the carbonate type, in particular an alkali metal carbonate, for example sodium, or a hydroxycarbonate chosen from hydrotalcite and dawsonite. According to a preferred embodiment of the process of the invention, the addition of the adsorbent during the final neutralization phase is carried out in powder form, preferably with an average particle size of the order of 10 to 20 microns . Other characteristics and advantages of the present invention will emerge from the description given below with reference to the appended drawings which illustrate an embodiment and an embodiment thereof which is devoid of any limiting character. In the drawings: FIG. 1 is a diagram which illustrates the various steps specified above of the process which is the subject of the present invention, and

Figure 2 is a schematic representation of an installation implementing such a method.

In this installation, it is noted that the final neutralization additive, which ensures the fixing of heavy metals, is added, in powder form and according to an average particle size of the order of 20 microns, to the liquid effluent pre-neutralized in an enclosure maintained with stirring. In this same device, the solid concentrate / permeat separation is carried out in an ultra or membrane microfiltration module which gives excellent results, in particular a very good quality of the permeat and a low volume of the solid materials generated. In addition, the use of such modules makes it possible to avoid the addition of coagulants, for the solid / liquid separation. The concentrated solid, after optional addition of polymer, is subjected to filtration, for example using a filter press, or to a centrifugation treatment in order to recover the solid materials containing the heavy metals which are then vitrified. The liquid from the filtration stage is recycled at the head of the installation.

According to the invention, the pH is regulated at the end of the final neutralization step by acting on the concentrations of the base (NaOH) used during the pre-neutralization step and / or of the adsorbent additive. used during the final neutralization step.

The results obtained by the present invention, which will emerge from the reading of the test reports, are the following: - elimination of heavy metals whose content goes from 185 mg / l to less than 10 mg / l,

elimination of metals: Fe, Cu, Ni, Cd, Pb, Zn, when the pH, at the end of the final neutralization step reaches at least the value 8,

- elimination of aluminum when the pH reaches at least the value of 7.5, but does not exceed the value of 9, in the final neutralization step;

the mercury can be eliminated by providing an additional loop at the outlet of the separation module, this loop using for example a sulphide for this elimination;

- considerable reduction in the volume of precipitation in the form of sludge, which is rendered inert, which makes it possible to store or reject solid waste in a natural environment, without danger to the environment; - reduction of around 20% in the cost of treatment.

Examples of implementation of the process of the invention have been given below confirming the results obtained, mentioned above.

In these examples, synthetic waters representative of washing water from smoke from household waste incinerators (OM) and chlorinated (Di-CI) or sulphated (Di-SO ₄ ) industrial waste were treated in the laboratory. Table 1 given in the appendix specifies the compositions of these three types of water covered by said tests.

The procedure common to all the tests was as follows:

1 / Preparation of synthetic water of the type considered in accordance with Table 1, using Miilipore water added with salts of metal chlorides and

CaCl ₂ , which gives an acid solution with a pH below

2;

2 / Pre-neutralization of this synthetic water by adding sodium hydroxide so as to obtain a pH of the order of 4, which eliminates iron and aluminum by precipitation of their metal hydroxides.

3 / Addition of the final neutralization additive to bring the pH to the desired value, in accordance with the process set out above (the various pH values tested are indicated below).

In this final neutralization step, additives were used as described in the RHONE-POULENC patent application mentioned above, that is to say respectively:

. a composite adsorbent consisting of a mixture (in mol%) of clay

(25%) and carbonate (75%) and

. a silicate composite adsorbent consisting of a mixture (in mol%) of clay (25%) of carbonate (37.5%), and of silicate (37.5%). The minimum quantities of the materials thus added in pulverulent form and according to the granulometry mentioned above, to the pre-neutralized synthetic waters are based on the stoichiometry = number of carbonate + silicate functions = number of moles of total metals (Cu + Ni + Cd + Pb + Hg + Zn) + number of moles of Ca. Let be in mass:

4 g / l of composite

4 g / l of silicate composite

This addition is carried out with stirring as has already been specified, and it is found that the pH instantly rises to the values indicated in tables 2 to 7 below.

According to the invention, the regulation of the pH obtained at the end of the final neutralization step can be carried out by controlling the concentration of the base (NaOH) used during the pre-neutralization step, and / or the addition of the composite adsorbent used in the final neutralization step, as specified above.

The metals in the filtrate were then assayed, and the results obtained were recorded in the following tables 2 to 7, which give the results for different synthetic water compositions corresponding to table 1, and for the various adsorbents specified above. -above.

In Table 2: for OM + composite (initial pH = 7.2): we note that the pH must be further raised from this initial value (either by adding sodium hydroxide during the pre-neutralization step, or by adding the composite) up to 9, to eliminate the zinc, the elimination of aluminum then posing a problem. The optimal result is obtained for pH = 8.5. In Table 3: for OM + composite silicate (initial pH = 7.9), good elimination of all metals except mercury is noted. In Table 4: for Di-CI + composite (initial pH 5.6), it can be seen that the elimination of total metals is low for this pH value. After readjusting the pH (as indicated above: either by addition of NaOH in the pre-neutralization step or addition of composite in the final neutralization step), it is found that the removal of the metals is good for a pH close to 9, with a problem regarding the elimination of aluminum, the optimal pH value being around 8 (pH = 7.8). In Table 5: for Di-CI + silicate compound (initial pH 6.7), good elimination is observed at this pH value up to the pH value = 9, the optimal results being obtained for a value close to 8 (pH = 7.7). In Table 6: for Di-SO ₄ + composite (initial pH = 5.8), there is a slight elimination of metals for this pH value, and that the optimal value is located at PH = 8.5.

In Table 7: for Di-SO ₄ + silicate compound (initial pH 6.9) there is an optimal elimination at pH = 9.

In all cases, it is noted that the silicate compound is more effective for the removal of metals in all types of water for dosing at stoichiometry by bringing the pH to 8.5.

By way of comparison, it will be recalled that the standards for liquid discharges into the natural environment (which must naturally comply with the water used to wash the flue gases from incinerators released after treatment) are as follows: . Total heavy metals <15 mg / l

. Cr VI <0.1 mg / l

. Pb <1 mg / l

. As <0.5 mg / l

. CN <0.1 mg / l

. Cd <0.2 mg / l

. Total Hg <0.05 mg / l.

In all cases, with an optimal choice of pH at the end of the final neutralization step, taking into account the additive for neutralization and fixing of heavy metals used, the invention makes it possible to meet the standards relating to the elimination of heavy metals. Furthermore, the small volume of solid discharges and their composition which makes them inert, facilitate their storage or their rejection in a natural environment without risks for the environment. As regards mercury, it has been seen above that its elimination can be obtained using a specific loop after the separation step.

It remains to be understood that the present invention is not limited to the examples of implementation or embodiment described and / or mentioned here, but that it encompasses all variants thereof.

Claims

1 - Process for the elimination of heavy metals contained in liquid effluents, in particular in water for washing fumes from incinerators of household or industrial waste, characterized in that it consists of:

- subject the liquid effluent containing the heavy metals to be eliminated to a pre-neutralization, in particular using sodium hydroxide, so as to raise the pH of the liquid effluent by a value of the order of 1 to about 4, which precipitates the metal hydroxides;

- subject the liquid effluent thus pre-neutralized to the metered action of a final neutralization additive making it possible to bring the pH of the effluent to be treated to a value of the order of 9, this additive ensuring the fixing of heavy metals;

- separate heavy metals and;

- Filter the concentrated solid from the separation step to obtain on the one hand the solid materials containing heavy metals which can then be subjected to vitrification, and on the other hand the liquid material. 2 - Process according to claim 1 characterized in that the additive for final neutralization and fixing of heavy metals is chosen from the group which comprises: zeolites, hydroxycarbonates, ion exchange materials, adsorbents such as in particular composite mixtures of clays + silicates and clays + silicates + carbonates. 3 - Method according to one of claims 1 or 2 characterized in that the additive for final neutralization and fixing of heavy metals is added in powder form.

4- A method according to any one of claims 1 to 3 characterized in that the concentrated solid / permeat separation is carried out using ultra modules or membrane microfiltration.

5- Method according to any one of the preceding claims, characterized in that a pH regulation is carried out at the end of the final neutralization step by acting on the concentrations of the base used in the pre-neutralization step and / or the additive used during the final neutralization step. 6 - Method according to any one of the preceding claims, characterized in that the mercury is removed using an additional loop, provided at the outlet of the separation module, this loop using in particular a sulfide for this elimination .