NL8403683A - METHOD FOR REMOVING RESPECTIVELY RECOVERY OF A SERIES OF HEAVY METALS FROM SEDIMENTS C.Q. SECONDARY SLUDGE. - Google Patents

Abstract

Recovering heavy metals from sediments such as sec. sludges, comprises acidifying the sediment or sludge to such a pH and adjusting the chloride ion concn. to such a level that one or more of the metals Cd, Zn, Co, Cu, Fe and/or Ni passes into soln. in the form of the metal ion-chloride complex, followed by passing the acidified suspension through an anion exchanger from which the chloride complex-bound metals can be separated and recovered.

Description

* £ N.0. 32791

Method for removing or recovering a series of heavy metals from sediments or secondary sludges, respectively.

The invention relates to a method for removing or reclaiming a series of heavy metals from sediments or secondary sludges, such as sludge contaminated with heavy metals, from ports and waterways, respectively.

In order to maintain the accessibility of this area for shipping traffic, approximately 23,000,000 m³ of sludge is dredged from the ports and waterways of the Dutch lower rivers area each year.

About 13,000,000 m 2 of this enormous amount of sludge is slightly contaminated and may currently be discharged into the sea. However, approximately 10,000,000 m 2 is too seriously contaminated to be discharged into the sea without causing serious environmental problems. As a result, dumping this sludge on land is the only possible alternative, but this is associated with major problems due to the high content of impurities, some of which easily rinse out. In addition to organic impurities such as oil, extractable organic chlorine compounds and polycyclic aromatic hydrocarbons, such sludge contains heavy metals, which are undesirable from an environmental hygiene point of view. In this context, cadmium is considered to be the relatively most threatening metal as it is mobilized very quickly from the sludge and can spread in the environment.

In view of the seriousness as well as the extent of the problems associated with contaminated sludge, an attempt has been made to find a solution for this in terms of their purification. For example, it is known from German patent application 82100349.8 to acidify heavy metal contaminated dredging sludge with concentrated hydrochloric acid to a pH of about 0. Under these conditions all metals, such as cadmium, dissolve. However, all lime also dissolves, so that considerable foaming is effected in this way. In this known method, the sediment is then separated from the liquid phase by decantation, after which the sediment is washed several times with water. According to this method, clean sludge is indeed obtained, but also an amount of waste acid, which is several times larger than the volume of the original sludge. The neutralization of the acidic aqueous phase is carried out, for example, with a carbonate, so that a voluminous precipitate of carbonate salts, such as cadmium carbonate, occurs. In summary, with respect to this known method, it can be stated that its economy leaves much to be desired, large quantities of waste acid are obtained 8403633 V.92 and a cumbersome solid / liquid separation method must be used.

In view of the above-mentioned drawbacks associated with this known method for cleaning heavy metal-contaminated sludge, an attempt has been made to develop a method which is free of these drawbacks.

It has been found that the above-mentioned drawbacks can be overcome if the sediments or secondary sludges are acidified to such a pH or the chloride concentration in the medium is raised to such a level that one or more of the metals cadmium, zinc, cobalt, copper , iron and / or nickel as a metal-on-chloride complex go into solution, after which the acidified suspension obtained is passed through an anion exchanger, from which the metals bound therein as chloride complexes can be removed and recovered.

The invention relates in particular to a method for removing or recovering cadmium and zinc, respectively, from sludge heavily contaminated with these metals, since these two metals are easily mobilized from sludge stored on land and can spread in the environment. for example, a danger to public health in view of its high toxicity and because of its accumulation upon absorption into the living cell.

Using the method according to the invention it is possible to recover a series of heavy metals, namely cadmium, zinc, cobalt, copper, iron and nickel, by acidification and chloride complex formation. The formation of chloride complexes of Cd and Zn already takes place at low chloride concentrations (chloride concentration: ^l mol / l), while the other heavy metals Co, Cu, Ni and Fe only fully form at higher chloride concentrations (chloride concentration 2 2 mol / l) complex. In addition to a higher chloride concentration, a lower pH must also be used with the latter metals.

An important advantage of the process according to the invention is the fact that the large amount of calcium ions obtained by dissolving lime no longer poses a problem for the recovery of the above-mentioned heavy metals. The extraction of these heavy metals with the aid of cation exchangers (Wesselingh) always involved great difficulties, because due to the large supply of calcium ions from the sludge, they were largely loaded with calcium ions, so that the use of a large excess of cation exchanger was necessary. . Since the above-mentioned heavy metals such as cadmium and zinc are in solution as a chloride complex and are recovered by an anion-exchanger, this "calcium problem" has been overcome.

- «1 P

. r * «3

In general, it can be stated that the method according to the invention relates to the acidification of sediments or secondary sludges and, respectively, the increase of the chloride concentration in the medium at such a height that one or more of the metals contains cadmium, zinc, cobalt, copper, nickel or iron dissolve as metal ion-chloride complexes, after which the chloride complexes are collected using an anion exchanger. Acidification takes place in view of the desired chloride concentration, preferably with hydrogen chloride, in particular with concentrated hydrochloric acid. The chloride concentration required for the chloride-10 complex formation can be adjusted with a water-soluble chloride, in particular sodium chloride.

Many exchangers known from the prior art can be used as anion exchanger, in particular strongly basic anion exchangers.

Since it is often sufficient to remove the cadmium and zinc, which are the most mobile metals, from contaminated sludge intended for storage on land, it is preferable to acidify to a mild pH in the range of 2-4, for example to a pH of about 3. Under these conditions, only cadmium and zinc dissolve and the less mobile heavy metals such as Cr remain quantitatively in the contaminated sludge.

More specifically, with regard to dredging sludge originating from ports and waterways, it can be pointed out that heavy metal impurities are concentrated in the fractions with the smallest particle size. For example, 70-80% of the heavy metals are concentrated in the fraction with a particle size of less than about 20 µm. In view of this large concentration of the heavy metals in the fraction with a particle size of less than about 20 µm, it is advantageous to fractionate the sludge into a fraction with a particle size of greater than about 20 µm and a particle size prior to the method according to the invention. fraction with a particle size of less than about 20 µm. Such a fractionation of dredging sludge can advantageously take place by hydrocyclonage. Namely, by hydrocyclonage, the heavy metal-contaminated sludge is separated into a relatively clean partial stream (coarse fraction with a particle size of greater than about 20 µm) and a relatively heavily contaminated partial stream (fine fraction with a particle size of less than about 20 µm). The fraction thus obtained with a particle size of less than about 20 µm can then be subjected to the method according to the invention.

The above-mentioned hydrocyclonage method could be carried out, if the dry matter content of the dredged sludge to be treated amounts to a maximum of approximately 30%. When this dry matter content is more than 30%, this content can be reduced to the level desired for hydrocyclonage by dilution with water.

Fig. 1 shows a process diagram for the recovery of the easily washable metals Cd and Zn, which metals must be removed in advance on storage of sludge heavily contaminated with these metals. The contaminated sludge supplied is first fractionated by sieving the sludge and then subjecting it to hydro-hydrogenation. Sieves remove the coarse components from the sludge to prevent possible blockages and wear of the pumps and the hydrocyclones. After sieving, the sludge is separated by hydrocyclonage into a fine (ca. approx. 20 µm) and a coarse fraction () approx. 20 µm).

Since, as mentioned, the heavy metals in the sludge are concentrated on the smallest particles, for example, hydrocyclonage concentrates about 70-80% of the heavy metals (in the fine fraction). The amount of dredge sludge to be purified is approximately halved by hydrocyclonage (on a dry matter basis).

After the above fractionation, the fine sludge fraction is subjected to an acid extraction. In an acid medium with a pH of 3 or 4, Cd and Zn of the sludge particles are desorbed in the liquid phase. This acid extraction is carried out batchwise in a stirred tank. The hydrochloric acid is metered in steps to reduce acid consumption. For example, to maintain a continuous purification process, a second acid extraction tank can be included in the process.

The mobilized heavy metals Cd and Zn form, after the acid extraction with hydrochloric acid in a chloride medium, negatively charged chloride complexes. These negatively charged chloride complexes are removed from the liquid phase with strongly basic anion-30 exchangers. Here, the sludge suspension is directly treated with the anion exchangers, because separation of the sludge particles (20 20 pm) from the liquid phase by filtration on a technical scale is a laborious and expensive process. The anion exchange process is performed in a fluid bed column. In the fluidized bed with the anion exchanger particles, the sludge particles are transported without problems. Due to the large difference in particle size between the anion exchanger particles (^ 500ym) and the sludge particles (^ "20ym), a fractionation of the sludge suspension in the floating bed is impossible. Due to the small density difference (200-300 kg / m 2) between anion exchanger and the upwardly flowing sludge suspension 8403683 »5, the anion exchange bed is homogeneously fluidized, with the result that the axial mixing in the floating bed will be small. The dust transfer process in a homogeneous fluidized bed can in principle be regarded as a counter-flow process. At the bottom of the floating bed is a zone 5 with loaded exchanger particles, while the lower-loaded and unloaded exchanger particles are located in an above zone.

When the anion exchange bed is largely loaded with Cd and Zn, the bed is eluted. During the elution, the column is operated as a packed bed and selective elution separates Cd and Zn. Cd is eluted with demi water and Zn with dilute hydrochloric acid (e.g. 0.005 M HCl). The concentrations of Zn or Cd in the elution fluid are very low and concentration of Zn or Cd can take place on a packed cation exchange column.

To maintain a continuous purification process, during the regeneration of the anion exchanger, it is possible to switch to a second fluid-bed column.

After treatment of the sludge suspension with anion exchangers, the sludge suspension is neutralized before deposition. Neutralization is probably not even necessary, because the acidity of the sludge suspension is continuously reduced as a result of the lime dissolving in the sludge dissolving.

The symbols a ~ p and 1-6 shown in Fig. 1 have the following meanings: a Supply of the contaminated sludge.

25 b Coarse sludge fraction after sieving, c Finer sludge fraction after sieving, d Underflow after hydrocyclonage,} approx. 20 pm. e Overflow after hydrocyclonage, approx. 20 µm. Concentrated hydrochloric acid, before the acid extraction.

30 g Fine sludge fraction after acid extraction.

h Fine sludge fraction after anion exchange, cleaned for Cd and Zn. i Fine sludge fraction after neutralization, j Lye, for neutralization of the acid sludge flow.

k Elution liquid (0.005 M HCl) for the recovery of Zn from the anion exchanger.

1 Elution fluid (Demi water) - for the recovery of Cd from the anion exchanger.

m Elution current contains Zn or Cd depending on the elution fluid. n Elution current after cation exchange, suitable for recirculation.

40 o Elution fluid (2M HCl) for the recovery of Cd or Zn from the 8403683 V t 6 cation exchanger.

p Elution current contains Zn or Cd depending on the loading of the cation exchanger, it can be loaded with Zn or Cd or both.

5 1 Vibrating screen 2 Hydrocyclone (s) 3 Acid extraction basin or system 4 Fluid bed column, with a strongly basic anion exchanger.

10 5 Packed-bed column, with a strongly acidic cation exchanger.

6 Neutralization basin or system.

The invention is further elucidated by means of the example below; however, this example should in no way be construed as limiting.

The example below focuses on removing the heavy metals cadium and zinc from harbors with class III (model sludge 20 1) and from class IV (class III with chromium contamination) (see model sludge 2).

The most important data for both model sludges are given in table A below.

25 TABLE A

____________________ model sludge 1_model sludge 2 dry matter content% 24 26

Cd mg / kg ds 14.1 15.5 30 Cr mg / kg ds 260 4420

Zn mg / kg d.s 1016 1790 lime g / kg d.s. 100 20 class_ΠΙ_IV_ 35

Particle size fractionation is by hydrocyclonage. A fraction with a particle size of less than, for example, 20 µm is obtained, which contains the majority of the heavy metal impurities and a relatively clean fraction with a particle size of more than e.g. 20 µm.

8403683 7

More particularly, with respect to the hydrocyclones, it is reported that particles suspended in a liquid therein are separated due to differences in particle size, shape and density. The separation is caused by centrifugal forces acting in a rotating liquid flow. The suspension is separated into a coarse "heavy" fraction (the lower run) and a fine "light" fraction (the upper run).

In Fig. 2, a cross-section of a hydrocyclone is shown, with (7) the inlet opening, (8) the cylindrical portion, (9) the conical portion, (10) the overflow outlet, (11) the underflow outlet ("apex nozzle ") and (12) display the" vortex finder "of the hydrocyclone. Fig. 2a shows a top view of the hydrocyclone.

Hydrocyclonage is a relatively inexpensive technique and extremely suitable for large capacities.

Fig. 3 shows a schematic arrangement according to which the fractionation for dredging sludge can be carried out using a hydrocyclone. In this schematic arrangement, (13) is a storage vessel with the sediment, (14) a pump, (15) the hydrocyclone, (16) the overflow drain, (17) the underflow drain, (18) an agitator and 20 (19) a bypass -line. According to this arrangement, the sludge is fed from a storage vessel with a pump through the hydrocyclone. The upper and lower courses of the hydrocyclone are returned to the storage vessel. The pressure at the input of the hydrocyclone is adjusted with a bypass. Settling in the storage vessel is prevented as a result of the recirculation. The main process conditions during the hydrocyclone experiments are shown in Table B:

TABLE B

Import dry matter content: + 10%

Hydrocyclone diameter: 15 mm 30 Input diameter: 3.0 mm

Vortex finder diameter: 3.0 mm

Apex nozzle diameter: 1.3 mm

Pre-pressure: 2.5-3.0 Bar

Three streams are important for the experiments: the inlet, the upper course (fine fraction) and the lower course (coarse fraction). Once the desired pre-pressure has been set, the relationship between these three flows is fixed. When a "stationary state" has been reached, the upper and lower sections are collected separately for a certain period of time, after which the cleaning method according to the invention can be applied.

8403683 * * 8

Tables C and D below list the results of the hydrocyclonage experiments of model sludges 1 and 2, respectively.

The heavy metal concentration factor i is defined as the ratio of the amount of contamination in the overhead to the amount of contamination in the input.

TABLE C

mass flow rate dry matter dry matter dry matter content X flow ratio r «tl, r -ii underflow / JM Lke-h J input [*] input 201 9» 8 19 »7 overflow 166 5 * 9 9 * 8 49.2 underflow 35 27 »1 9.5 concentration ci of concentration heavy metal i factor f ^ [$] [mg / kg ds] _ ° Cd cCr cZn fCd fCr f2n input .14.1 260 1016 upper flow 22.8 485 1562 80 89 76 underflow 6.4 161_589_ 8403683 9 5

TABLE D

mass flow rate dry matter dry matter dry matter content X flow rate ratio f -11 fir * .ii underflow / | ks * b 1 M [fcs.ï 1] input [56] input 230 9.0 · 20.7 overflow 183 5.2 9.5 50.0 underflow 48 21.7 10.4 concentration of concentration of the heavy metal i factor f ^ [$] [mg / kg ds] __ cCd cCr cZn fCd fCr fZn input 13.5 3260 1495 overflow 20.4 5316 2184 89 75 67 underflow 7.2 1248 861 From the results shown in Tables C and D it appears that with an import with a dry matter content of 9-10%, via a hydrocyclone with a diameter of 15 mm, approximately 50% of the input, on a dry matter basis, ends up in the headwaters and Cd, Cr and Zn are concentrated in the headwaters. For model sludge 1, about 75-90% of the heavy metals Cd, Cr and Zn from the input are concentrated in the headwaters; the concentration factor for model sludge 2 is slightly lower, approximately 65-75%.

The effect of the hydrocyclonage is illustrated in Tables E and F below.

8403683 V TABLE E (model sludge 1) 10 X = 5.1 t

Ccd = 22.8 mg / kg a.s.

- 166 kg / h * c _ 455 mg / kg d.s.

X = 9.8 ^ I e ™ 1 * = 1562 mg / kg a.s.

cCd = 14.1 mg / kg a.s.P Zn-J

cCr = 260 mg / kg a.s. “201 kg / T7. I X = 2V * cZn * 1016 mg / kg a.s.y ^ - 6.4 mg / kg a.s.

- * 35 .kg / li >> _ ι5ΐ mg / kg a.s.

Cr.

Cr, = 589 mg / kg a.s.

An_______ - TABLE F (model sludge 2) X = 5.2 ccÉ cc- = 20.4 mg / kg d.s.

lx. 9.0 * I | -183 kg / h | lc ^ r = 531g mg / kg a.s.

ecd = 13.5 mg / kg d.s. I CZn = 2184 ms / kg d.s.j cCr = 3260 mg / kg d.s. ”230 kS / h> M χ = 2 //? ? czn B 1495®gAs a * sV Cnj = 7.2 mg / kg a.s.

1—48 kgA> Ccr = 1248 mg / kg a.s.

cZn 85 d, s *

The fine fraction with a particle size of less than about 20 µm obtained by the above method is then subjected to the acid extraction according to the invention. The tests were carried out in beakers stirred with a magnetic stirrer and concentrated hydrochloric acid was used as the acid.

The acid extraction tests were carried out at different acid degrees, namely at a pH of 1, 2 or 3. The concentrated hydrochloric acid is added stepwise to the sludge suspension until the desired pH is reached. For model sludge 1, a strong foam development occurs after addition of the acid, because considerable amounts of lime dissolve. For model sludge 2 hardly any foaming occurs, but a strong H2S odor is noticeable.

8403683> <11

After an extraction time of about 3 hours, the treated sludge is subjected to purification with anion exchangers.

Tables G and H below list the results of the acid extraction experiments of model sludge 1 and 2, respectively.

The removal efficiency i for the heavy metal i by acid extraction is defined as the amount of impurity removed from the sludge by acid extraction relative to the amount of impurity in the starting material.

TABLE G (model sludge 1) ~ pH acid concentration cl of disposal consumption heavy metal i yield h.

in the sludge. 1

[mg / kg d.s.] [$ J

kg d.s.

cCd cCr cZn ^ Cd \ r 7 0 22.8 465.1562 000 3 0.084 0.90 * 372 96 * 76 2 0.105 0.80 * 247 96 * 84 1 0.250 0.77 374 182 97 20 88 8403683 TABLE H ( model sludge 2) 12 pH acid concentration ci of removal consumption heavy metal -i efficiency ii. in the sludge.

^ HC- [mg / kg d.s.] £ kg d.s.

cCd cCr cZn \ d ^ Cr ** Zn 7 0 20.4 5316 2184 0 0 0 3 0.024 1.1 * 519 95 * 76 2 0.051 0.96 * 432 96 * 80 1 0.341 0.90 2774 292 96 48 87

The acid extraction tests of the model sludges 1 and 2 show that cadmium is almost completely removed (96%) from the sludge even at a low acidity (pH = 3). An increase in the acidity has little effect on the amount of Cd extra removed from the sludge. For zinc, depending on the pfl, this metal is 75-90% removed from the sludge.

The fine fractions of model sludges 1 and 2, which have been treated with hydrochloric acid in the acid extraction, are then passed through an anion exchanger (D0WEX-1-X8). The anion exchanger is first conditioned for use. This conditioning consists of a sieving operation (500ym) and a regeneration of the anion exchanger in the chloride form. The ion exchanger is screened to remove any grit and very small ion exchanger particles, so that after the experiments have ended the ion exchanger particles can be easily separated from the sludge slurry (by sieving with a 250 µm mesh sieve). When the acidified sludge suspension is passed through the anion exchanger, the following process parameters apply: - chloride concentration; ¢¢ ^ - = 1 mol / 1.

20 - amount of anion exchanger relative to amount of sludge on a dry matter basis; 2.2 kg exchanger / kg d.s. (excess, batch experiment) - stirring speed; 300 rpm.

- temperature; 18-22 ° C.

The residence time of the anion exchanger in the sludge slurry is 5.30 or 120 minutes and the pH during the anion exchange process is 1, 2 or 3.

The tests were carried out batchwise. After addition of the anion exchanger, mixing is carried out by means of a magnetic stirrer. During the experiments, "mild" was stirred to prevent any wear of the anion exchanger. After a certain residence time of the exchanger in the sludge suspension, the ion exchanger particles are separated by seven (250 µm) from the sludge suspension.

Tables J and K below list the results of the tests on model sludges 1 and 2.

The extraction efficiency i of the heavy metal anion exchanger 1 is defined as the amount of impurity transferred from the liquid phase to the anion exchanger relative to the original amount of impurity in the liquid phase for treatment with the anion exchanger.

15 The symbols used in the tables are briefly explained below: T *: residence time of the exchanger in the sludge suspension.

X: dry matter content of the sludge suspension.

ci, s: concentration of the heavy metal i, in the dry matter.

20: concentration of the heavy metal i in the liquid phase.

ci, 1J concentration of the heavy metal i, in the liquid phase on a dry matter basis.

8403683 TABLE J (model sludge 1) 14 PH τ X ° Cd, s cCd, 1 cCd, l led cZn, s cZn, l cZn.l ^ Zn mg mg mg mg mg mg ^ m: Ln ^ kg d.s. 1 kg ash ^ kg ds 1 kg ds 3 o 4.30 0.90 1.30 29.0 0 372 71.4 1590 0 3 5 3.91 * 0.23 5.6 81 * 23.9 587 63 3 30 3.86 * 0.12 2.9 90 * 17.1 425 73 3 120 3.47 0.30 0.07 1.9 93 316 13.4 374 '76 2 0 4.37 0.80 1. 33 29.1 0 247 73.4 1608 0 2 5 4.10 0.29 0.32 7.6 74 183 29.4 689 57 2 30 4.01 0.27 0.12 2.8 90 170 16, 4 399 75 2 120 3.99 0.39 0.07 1.7 94 178 13.5 324 80 1 0 4.35 0.77 1.08 23.8 0 182 74.6 1641 0 1 5 3.34 * 0.07 1.9 92 * 10.6 309 81 1 30 3.54 * 0.02 0.4 98 * 2.6 72 96 1 120 3.32 0.10 0.01 0.2 99 128 2 , 3 66 96 84 0 3 S 8 3 * TABLE K (model sludge 2) 15 pH τ X cCd, s cCd, l cCd, l ^ Cd cZn, s cZn, l cZn, 2 min + -.ss - ss. «G— $ —SSss - ^ kg ds 1 kg ds kg ds 1 kg ds“ 1 (Γ 4.50 1.10 Ö, 86 18.4 Ö 519 89.4 1698 o ”3 5 3.95 * 0, 15 3.8 79 * 33.9 824 57 3 30 3.71 * 0.06 1.6 91 * 17.4 453 76 3 120 3.96 0.30 0.03 0.8 96 136 11.9 288 85

2 O 4.26 0.96 0.83 18.6 O 432 87.5 1966 O

2 5 3.57 * 0.11 3.0 84 * 22.3 601 69 2 30 3.56 * 0.05 1.3 93 * 11.9 322 84 2 120 4.22 0.20 0.01 0 , 4 98 91 7.4 167 92

1 O 3.85 0.90 0.79 19.8 O 292 86.2 2154 O

1 5 3.97 * 0.05 1.3 94 <11.4 285 87 1 30 3.80 0.10 0.01 0.4 98 63 4.6 117 95 1 120 3.70 0.10 0, 01 0.3 99 55 3.7 96 96

Due to dust transfer from the heavy metal chloride complexes in the liquid phase to anion exchanger particles, the concentrations of heavy metals in the liquid phase decrease sharply.

An interesting side effect is that the concentrations of heavy metals in the sludge particles themselves decrease. This is presumably due to a shift in the balance between the heavy metal in solution (which is absorbed by the anion exchanger) on the one hand and the heavy metals present in the sludge particles.

Finally, the sludge suspension from the anion exchanger can be neutralized before deposition. Such a neutralization 8403683 16 is not necessary since the acidity of the sludge slurry is continuously reduced as a result of the lime dissolving in the sludge becoming more and more dissolved until an almost neutral value is reached.

84 0 3 8 8 3

Claims (10)

Method for removing or recovering a series of heavy metals from sediments or secondary sludges, respectively, characterized in that the sediments or secondary sludges are acidified to such an I® or to bring the chloride concentration in the medium to such a height that one or more of the metals cadmium, zinc, cobalt, copper, iron and / or nickel go into solution as a metal ion-chloride complex, after which the acidified suspension obtained is passed through an anion exchanger, from which the metals bound as chloride complexes can be removed and won. 2. Process according to claim 1, characterized in that the sediment or secondary sludge is acidified with hydrogen chloride. 3. Process according to claim 1 or 2, characterized in that the sediment or sludge is acidified with concentrated hydrochloric acid. 4. Process according to one or more of claims 1-3, characterized in that the chloride concentration required for the metal ion chloride complex formation is adjusted by adding one or more (earth) alkali metal chlorides to the medium. Process according to claim 4, characterized in that the chloride concentration required for metal ion chloride complex formation is adjusted by adding sodium chloride to the medium. 6. Process according to one or more of claims 1-5, characterized in that the sediment or secondary sludge is acidified to a pH in the range of 2-4. Process according to one or more of Claims 1 to 6, characterized in that the metal-contaminated sediment or secondary sludge is suspended as a suspension by means of a hydrocyclone to some extent with heavy metal-loaded particles with a particle size of more than about 30 20 µm and heavy metal contaminated particles with a particle size of less than about 20 µm. Process according to claim 7, characterized in that a suspension with a dry matter content of more than about 30% is diluted with water to a dry matter content of less than about 30%. Process according to claim 1, characterized in that a strongly basic anion exchanger is used as the anion exchanger. Process according to one or more of Claims 1 to 9, characterized in that the suspension product guided through the anion exchanger is neutralized. 40S 4 0 3 6 8 3 *******