AU1744599A - Oxidising leaching of contaminated sludge containing iron with separation of zinc and lead - Google Patents

Abstract

The invention concerns a method for separating non-ferrous metals such as Zn and Pb from sludge containing iron, characterised in that it consists in the following operational steps: non-selective lixiviation of the iron-containing sludge in an acid oxidising medium with a pH less than 2 in a reactor (10) to obtain a lixiviated suspension; a first separating step wherein the lixiviated suspension is separated on a filter (6) into a solid fraction (16) and a solution (20); an oxidising step (9) wherein the solution is oxidised by an oxidising agent (21), thereby converting Fe<++ >in the solution into Fe<+++>; re-using the oxidised solution (4) in the lixiviation reactor (10), thereby precipitating Fe<+++> in the reactor and eliminating it from the solution during the first separating step (16) while the acid bound in the form of FeCl3 is recuperated.

Description

OXIDISING LEACHING OF CONTAMINATED SLUDGE CONTAINING IRON WITH SEPARATION OF ZINC AND LEAD Object of the invention The present invention concerns a process for the treatment of contaminated sludge containing iron, such as residues from the steel industry, more particularly to separate non-ferrous metals from sludge in order to permit the re-use of the treated sludge in the steel industry, by way of raw materials. Technological background at the basis of the invention The production of steel by the traditional method (reductive fusion of iron ores in blast furnaces or the fusion of scrap iron and iron sponges in electric furnaces) involves the formation of a residue contaminated by zinc and containing iron. The content of zinc in such a material is variable and depends on the process used and the starting materials. Generally speaking, a separation of materials with a high content of zinc from materials with a low content of zinc is possible. Materials with a low content of zinc can be recycled in production immediately. This technique is called "internal recycling". It causes the formation of a dust enriched with zinc which must be treated by other techniques.

W

rrT' Another process is that of vitrification or inertisation. By this process, material which is not economically recyclable is treated in such a way that the toxic residues present in the dust are incorporated into other products so as to obtain final products which are inert in the presence of water and air. These products can then serve as base products in road building or as glass or ceramic material. Examples of such processes are the vitrification of mixtures of the dust with silicates heating between 800 and 14000C, which causes the formation of glass products. Another process of the same type consists in mixing the dust with alumino-silicates, lime and other additives, causing a reaction of the heavy metals in the dust with the additives in such a way as to form a matrix of silicate of calcium and aluminium, which presents a structure similar to concrete and which can be used as a ground stabiliser. Zinc is not recycled but fixed in the matrix. Another type of process can be described in a general way as a pyro-metallurgical procedure. These procedures aim to produce pure metallic ZnO or Zn. The best known example is the so called WAELZ Rotary Kiln process. The materials to be treated are simultaneously introduced with reducing agents and additives into a furnace. In the reduction zone of the furnace, ZnO is reduced and evaporated. In the oxidising zone of the furnace, zinc is re-oxidised to form a zinc oxide which is recovered as a raw material in a gas purification installation. The difficulty of this process is that the ZnO which is produced contains too much chlorine and fluor, and in some cases, too much Cd and Pb. This results in - D A L/,I 1 NV 0\ a need for complementary treatment before proceeding to fusion in an installation like the Imperial Smelting Furnace, for example. A final type of treatment process can be described as hydro metallurgical processes. These processes are characterised by the presence of a leaching operation. The leaching agents already described are NaOH (process S.E.R.H.-CEBEDEAU),

NH

3 (processes UBC Chapparral and EZINEX) and H 2

SO

4 (Modified Zincex Process) as well as some organic acids. The most commonly used are NaOH and H 2

SO

4 , but these agents present some specific problems: on using NaOH, one must use a strongly concentrated solution to obtain a sufficient solubility of Zn, whereas on using H 2

SO

4 , silicates and ferrites are hardly lixiviable, and in the case of certain ores, a formation of a silica gel occurs, involving difficulties of separation. State of the art In the document LU-87 535, there is described a process for the treatment of materials containing heavy metals, in particular steel industry residues, by acidic leaching. The characteristic of the invention is to use as a reagent acids that are rich in Fe+++ ions and poor in Fe++ ions. It may therefore comprise a preliminary step consisting in submitting an acid solution rich in Fe++ ions to an oxidisation operation so as to oxidise these ions and produce a solution rich in Fe+++ ions. It is proposed V l l N N O in this case, with a view to recovering steel industry residues, the use of an exhausted pickling bath, as these baths are rich in Fe++ ions. This technique implies the presence of such an acid solution rich in Fe+++ ions with materials to treat containing heavy metals so as to cause the precipitation of at least a part of the Fe+++ from the said acid solution and also the dissolution of the heavy metals with a view to producing a second solution. Measurements are taken at regular intervals of the pH of this second solution in order to maintain a pH that is particularly to 2. The pH is then controlled to a value contained between 3 and 5 by the addition of an alkaline substance, then the residual Fe++ is oxidised into Fe+++ with the help of an oxidising agent and a new selective precipitation of the residual iron in solution is obtained. Next, the solid phase and the liquid phase are separated, and a reagent is added to the liquid phase, causing the formation and the precipitation of an insoluble compound of said heavy metals, either simultaneously or successively. After each precipitation operation of an insoluble compound, the residual liquid phase and the solid phase containing the said insoluble compound are separated. The choice of pH of the order of 2 which is quoted, is meant to reduce as much as possible the re-dissolving of the iron. The regulation of the redox potential by adding Fe+++ is meant to keep the lead in solution. In the described technique, the dissolved iron is precipitated in a second reactor at the time of the increase of pH, which involves the loss of reagents.

In the document AT-400 928-B, there is proposed a use of ion exchangers, and more precisely of cation exchangers, selectivity being reached by modifying acidity rate. The document GB-2 128 797-A also proposes a cation exchanger to recover non-ferrous metals. The document Database WP1, Section Ch, Derwent Publications Ltd., London, GB; Class D15, AN 73-72860U XP002074793 & JP 48 055 815 A (Momozaki J) describes a treatment technique for dust originating from the steel industry with residual HCI oxidised with chlorine gas or nitric acid. After neutralisation and separation by filtering of the iron hydroxide, the filtrate is treated with metallic zinc to permit precipitation and filtration of heavy metals. Thereafter, the filtrate is neutralised to recover the zinc hydroxide. The document Database WPI, Section Ch, Week 12 1993, Derwent Publications Ltd., London , GB; Class J01, AN 93-098837 XP002074794 & SU 1 725 949 A (Balkhashmed Prod. Assoc.), April 15, 1992, describes the release of metallic cations under the effect of a desorption solution. Aims of the invention The present invention aims to propose a new hydro-metallurgical process intended for the elimination of Zn and Pb as well as other contaminants such as the alkaline or alkaline-earth metals from the sludge, which allows the recovery of iron in sintering installations while the non ferrous metals can be purified and re-utilised.

Main characteristics of the invention The essential feature of the invention is a process for leaching contaminating metals such as Zn or Pb in order to separate them from sludges containing iron, comprising the following operation steps: - a non-selective step of leaching the sludge containing iron in an acid oxidising medium at a pH lower than 2 in a reactor so as to obtain a leached suspension; - a first step of separation in which the leached suspension is separated in a solid fraction and a solution; - an oxidisation step in which the solution undergoes oxidisation by an oxidising agent, causing the conversion of Fe++ present in the solution into Fe+++; - re-use of the oxidised solution in the leaching step, causing precipitation of the Fe+++ in the reactor and its elimination from the solution in the course of the first step of separation, while the bound acid in the form of FeCl 3 is recovered. The re-use of the oxidised solution in the step of leaching is remarkably advantageous: the solution is at low pH, which reduces the consumption of acid, while the bound acid in the form of FeCl 3 is recovered in the reactor. Moreover, the solution must not undergo neutralisation. The Fe++ which passed in solution in the reactor is converted into Fe+++ in the oxidisation step, causing precipitation in the reactor and the possibility of qre-use in sintering installations. Furthermore, there is a lower consumption of water per tonne of treated sludge. The first step of separation can be achieved by filtration. The leaching step can include the addition of HCI and/or FeC 3 . The pH in the step of leaching is lower than 2, preferably lower than a value of the order of 1.5, with values up to 0.5 being possible. The process allows working at a low rate of acidity, because the acid used can be recovered by recirculation; in other words the costs in chemical reagents and the costs of neutralisation do not significantly increase. The quantity of iron which passes in solution is not of prime importance. The aim of the value of pH chosen is that about 95% of Zn and of Pb present pass into solution, the rest being recovered in sintering installations. The redox potential in the step of leaching is preferably higher than about 450 mV. According to a particular embodiment of the invention, the process includes a second step of separation, which includes a separation of the solution resulting from the first step of separation on at least one ion exchanger, with the concentration in Cl - ions in the solution being at least of the order of 1 M. Any oxidising agent can be used, with C12 being however preferred for an industrial application. The average time for treatment for the leaching step is preferably about 2 hours.

L

The concentration in Cl - in the solution is preferably comprised between 1.5 and 2.5 mol/l. This has an advantageous effect on the capacity of the ion exchanger. In fact, the higher the concentration in Cl - is, the better will be the ion exchange of Zn and Pb. However, the exchange of Fe ions, which is only 1 % for a concentration in chloride of 2 mol/l, increases somewhat when the concentration in Cl - exceeds 2 mol/1. This is to be avoided because of the fact that the separation becomes worse. The ion exchanger can be regenerated with water and/or an aqueous solution of metallic ions, in which case the product of regeneration (regenerate) which flows out from it is collected. This means that the ion exchanger can be regenerated with the "regenerate" thus obtained until the concentration in metal in the latter becomes too high. Therefore, it is not necessary to use acids or bases for regeneration, and it is not always necessary to use pure water, which reduces the water consumption of the installation. Starting with the regenerate, one can then eliminate from the solution the Pb which is present by means of "cementation" i.e. displacement by adding zinc powder; the metallic zinc can be recovered by electrolysis or the Zn(OH) 2 or ZnCO 3 can be recovered by means of a precipitation reaction. The invention will be described in more details with reference to the preferred embodiments in reference to the accompanying drawings. In the figures, identical elements having the same function have the same reference numbers. LU

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Description of the figures The figures 1 and 2 are schematic representations of the installations enabling the implementation of the invention. The figure 3 represents an embodiment of a reactor for the step of leaching. This reactor is used in a pilot installation. The figure 4 represents an embodiment of a filter to achieve the first step of separation. The figure 5 represents the quantity of Zn and Pb (g/i) in the outflow from the ion exchanger as a function of the treated volume (m/I). The figure 6 represents the evolution of the quantity of Fe++ in % and of the redox potential in mV as a function of the quantity of NaOCI fed in the oxidisation step expressed in litres NaOCI per litre of leaching liquid. The figure 7 represents the evolution of the redox potential (mV) as a function of the quantity of Ca(CIO) 2 added in the oxidisation step. The figure 8 describes the evolution of pH as a function of the quantity of Ca(CIO) 2 added in the oxidisation step. Detailed description of the invention The invention concerns the elimination of contaminating metals such as Zn and Pb from sludges containing iron by resorting to a non selective leaching, a separation to obtain a solid material and a solution, and the re use of oxidised solution. The invention will be described by referring to the examples. C CDi 'Ir_ A general schematic representation of the installation by which the present invention can be implemented is represented in figure 1. The sludge is brought in 1 into a reactor 10 simultaneously with the chemicals 2 (for example HCI) and a recirculated product 4, hereafter called "recirculate". In the reactor 10, the step of leaching occurs in an acidic oxidising medium. The leached suspension is then stored either in a buffer reservoir 25 (optional), or directly filtered on a band filter 6. To achieve a good filtration, a flocculant can be added (for example Zetag 32); this is done preferably in the product that overflows from the reactor. The separation on the band filter 6 (first step of separation) produces a solid material and a solution. The solution which contains all the dissolved metallic ions is brought to the ion exchangers 7 and 7', which fixes Zn and Pb in the form of complexes of chloride in the ion exchanger, while all the other metallic ions pass without being complexed. The Zn and Pb ions can be eliminated by carrying out a regeneration of the ion exchanger. The passing solution whose Zn and Pb ions have been separated then undergoes an oxidisation at 9 by adding an oxidising agent (for example chloride gas Cl 2 or NaOCI). The oxidised solution can be re-utilized as a "recirculate" 4 in the reactor 10. The oxidised iron in the form of Fe+++ will pass into solution in the reactor, which readily allows its recovery in the course of the filtration step in order to be re-used in sintering installations. The essential part of the invention is the leaching step. This can be achieved by adding HCI and/or FeCl 3 to the sludge. The leaching, that is to say, the passing in solution of metallic ions, is carried out at a pH lower than 2, preferably of the order of 1.5. Given that, generally speaking, contamination is due to Zn and Pb and that Zn constitutes a technological problem in the blast furnaces, the essence of the invention bears on the elimination of Zn from the sludge. Sludge compositions used in the examples The table 1 shows an average composition in dry matter emerging from two blast furnaces (A and B) and a lagooning of sludge. Blast furnace A Blast furnace B Sludge lagoon Dry matter content 13+2 C 25+5 21+5 S 2.1+0.6 2.1+0.7 Pb 1.02+0.02 Zn 5+2 5+2 4.43+0.08 Fe 14+4 14+5 13.5+0.3 Mn 0.11+0.05 0.11+0.04 0.15+0.02 Ca 13+4 13+5 4.6+0.1 Al 2+1 1.7+0.9 1.19+0.03 P 0.2+0.1 0.2+0.2 Mg 1+1 2+1 1.41+0.05 Si0 2 6+3 6+3 Na 0.13+0.08 0. 2+0.1 K 0.2+0.2 0.2+0.2 Table 1: Average composition (%) of dry matter in blast furnace sludge in blast furnaces A and B and in the sludge of a sludge lagoon.

A-

Example 1: Leaching with HCI or FeCl 3 of Zn,Pb, and Fe from the sludge emerging from a laqooninq influence of pH. The sludge emerging from a lagooning is leached with HCI in a trial installation according to figure 1. The leaching yields for different metals are represented in Table 2. The metal concentrations in the leaching liquid are represented in Table 3. pH Mg Al Ca Fe Mn Zn Pb 3.31 64 56 81 10 29 32 0.2 1.46 59 67 79 15 42 63 35 1.42 80 86 91 25 51 60 5 1.31 71.1 80.0 92.0 20.4 46.1 55.7 36.0 1.08 85.0 89.3 82.2 38.4 55.1 68.8 0.7 1.01 78.8 77.9 89.6 44.9 46.9 58.1 47.6 1.01 88.0 89.0 95.0 53.0 80.0 89.0 90.0 0.90 79.1 76.4 91.2 45.0 48.7 58.1 58.0 0.75 97.5 93.5 99.9 67.8 96.0 89.8 96.2 0.72 82.6 78.1 93.9 55.5 55.9 64.6 89.1 0.68 78.6 74.0 91.9 48.7 50.3 60.7 90.2 0.62 78.0 81.9 93.1 27.9 57.2 69.2 93.3 0.54 94.8 72.2 99.9 73.4 91.4 97.9 97.6 0.45 86.3 77.5 95.5 46.9 65.6 61.3 65.8 0.38 99.1 98.9 99.8 92.2 98.7 95.6 97.4 0.37 91.4 83.4 96.7 52.2 75.5 73.6 91.0 0.34 88.8 78.9 95.0 46.7 74.6 75.1 98.5 0.3 86.7 70.9 91.3 42.6 70.6 71.7 98.3 0.25 88.5 89.6 95.8 49.2 69.3 80.9 9.4

,;C

Table 2. Yields (%) as a function of final pH when leached with HCl. pH Mg Al Ca Fe Mn Zn Pb 3.31 369 600 1260 746 16.2 786 1 1.46 297 566 1140 1370 27.2 1034 194 1.42 329 664 1086 1032 17.6 1007 17.1 1.31 255 517 1782 1024 17 897 179 1.08 271 660 844 1046 13 1822 5 1.01 247 432 554 1646 10 1426 368 1.01 343 576 1173 2304 35 1575 593 0.90 235 387 509 1545 10 1310 420 0.75 186 136 6772 460 22 522 70 0.72 219 349 437 1670 9 1291 688 0.68 205 322 402 1571 9 1201 643 0.62 381 665 1610 1800 25 1383 603 0.54 406 141 7081 1014 21 632 380 0.45 198 353 486 1659 12 1073 701 0.38 490 814 14460 2078 59 1392 256 0.37 155 260 371 1040 9 760 677 0.34 176 298 421 1410 13 916 681 0.3 190 269 418 1686 15 1009 707 0.25 328 816 952 1606 21 2564 80 Table 3: Concentrations (mg/) in the leaching liquid for different metals when leached with HCI in a trial installation. For a leaching by FeC 3 , exactly the same procedure has been used as in example 1, the only difference being that here FeCl 3 is used instead of HCl. The yields of leaching for this example are indicated in Table 4. The ATh result obtained for Fe should not be taken into consideration, considering the fact that it is falsified by the presence of Fe in the leaching medium. pH Mg Al Ca Fe Mn Zn Pb 6.02 60.1 0.1 60.7 11.3 41.8 15.5 0.1 5.53 68.0 1.0 73.2 24.1 46.3 41.0 0.4 5.43 69.3 0.8 71.3 26.4 47.4 44.3 0.3 5.16 79.6 2.0 84.4 24.5 63.8 53.2 1.4 4.78 78.9 3.6 84.8 25.0 55.9 58.9 3.2 4.08 82.8 12.2 88.2 27.3 63.1 71.2 6.1 3.88 82.8 54.3 87.7 51.6 70.5 79.6 73.0 3.82 86.7 37.1 96.8 28.1 67.8 87.4 54.9 3.74 88.2 45.4 92.9 29.8 74.9 87.9 67.0 2.53 89.6 82.3 97.4 28.5 72.1 95.5 94.6 1.87 87.3 83.8 97.0 47.7 73.8 96.9 97.0 1.82 90.5 91.8 95.0 80.8 86.6 98.1 99.7 1.78 90.5 83.0 97.3 92.7 79.5 96.7 96.9 1.63 99.3 99.2 99.4 97.5 99.7 99.7 100.0 1.55 88.7 93.3 92.2 82.7 92.7 98.4 99.7 Table 4. Yields (%) as a function of final pH when leaching with FeCl 3 . The results of the leaching by HCI or FeCl 3 allow to conclude that an oxidising acid medium (FeCl 3 ) gives better yields of leaching than a non oxidising acid medium (HCI).

\LI

Example 2 : Separation by means of an ion exchanger of the solution obtained after leaching An actual leaching liquid is brought to an anion exchanger (DOWEX SBR). Table 5 shows the composition of the leaching liquid. pH 0.4 Zn (mg/I) 2574 Ca (mg/) 16200 Fe (mg/I) 15300 Mg (mg/) 1486 Al (mg/1) 415 Pb (mg/I) 740 Table 5: Composition of a leaching liquid for an ion exchanger Figure 5 shows the concentration of Zn and Pb in the effluent in terms of the volume having circulated. At the start, Zn and Pb are completely eliminated. The other metals are not kept in the column. As the saturation of the column progresses, the concentration in Zn and Pb increases up to the value in the inflow and regeneration becomes necessary. The capacity for exchange depends highly on the concentration of Cl - in the leaching liquid. A value of about 1 to 2 mol/I of CI - is necessary for a good exchange. If the concentration in Cl - becomes too low, capacity will drop strongly. By increasing the concentration of Cl -, affinity for Fe+++ increases equally, but at a value Cl ~ 2N, only 1% of Fe+++ present is exchanged.

A-

IUV Example 3 : Influence of the concentration of Cl - on the capacity of the ion exchanger The concentration in Cl - is important for the quantity of Zn and Pb which can be separated from the solution in the form of complexes of chloride through binding on an ion exchanger. The theoretical capacity of the anion exchanger DOWEX SBR is 3.5 meq/g of dry resin. To evaluate the influence of the concentration of Cl -, measurement was made of the retention of the ion exchanger for different concentrations of Cl -. The results are represented in Table 6. Cl - (N) Concentration Zn (meq/g dry resin) 2.0 3.18 1.0 1.56 0.3 0.44 Table 6: Influence of the concentration in Cl - on the capacity of the anion exchanger. Example 4: Oxidisation of a liquid by leaching of NaOCI NaOCI is added progressively to the leaching liquid showing the composition of Table 7 (after ion exchange). After a few additions, the redox potential is measured and a sample is taken. In the sample, the ratio of Fe++/Fe+++ is measured. The result is represented in figure 6. When all the Fe++ is oxidised, a marked increase of the redox potential 24 is observed. This can be used for the regulation of oxidisation.

LU

PH 1.17 E (mV) 480 Mg (mg/I) 381 Al (mg/I) 1389 Ca (mg/) 1461 Fe 2 + (mg/) 2910 Fe 3 * (mg/) 4561 Mn (mg/) 41.5 Zn (mg/) 6.3 Pb (mg/I) 37.2 Cl (mo/) 0.72 Table 7: composition of the leaching liquid. Example 7 : Evolution of the redox potential and of pH when oxidisinq with Ca(CIO) 2 The redox potential and the pH are measured in a 150 mmol/l solution of FeSO 4 . This solution has a pH of 1.3 and a redox potential of -328 mV. H 2

SO

4 or NaOH is added to the solution to determine the influence of the pH of the solution on the oxidisation process. To 50 ml of this solution, there is added every 2 minutes, 2 ml of a solution of Ca(CIO) 2 . The concentration of the solution of Ca(CIO) 2 is 170 mmol/l. The results are represented in the figures 7 and 8. From the fact that an oxidised leaching liquid shows a pH and a redox potential that are appropriate, it can be re-used in the leaching step. It contains then essentially FeCl 3 . Lii Example 8 : Use of the band filter The band filter (Philippe Filter) used is that of figure 4 which will be described hereafter. The best parameters of the filter band are represented in Table 8. Concentration of flocculating agent (Zetag 32 (ppm)) 100-200 Speed of filter band (m/min) 1.1 Flow treated (I/u) Approx. 600 Thickness of filtration cake (mm) Approx. 5 Filtration cake VSG (%) 34.4±9.7 Table 8: Optimal parameters of the band filter Simulation of the procedure During the simulation process, all the steps of the process are carried out sequentially in the lab. By proceeding to several successive repetitions, it is also possible to simulate the re-use of the "recirculate". A step of the process is always a leaching, a filtering on a folded filter, an ion exchange and an oxidisation by NaOCL. There are implemented in all 5 experiments in different conditions such as the addition of FeCl 3 or of NaOCI as the agent of oxidisation at the time of reaction. The installation Description of the installation The installation used is shown in figure 2. rL The sludge originating from the blast furnaces in precipitator 1 presents a content in dry matter of 5%. This sludge is fed into the thickener 7 at the rate of 20 m 3 /h. The aim of this step is to increase the content in dry matter in the sludge, preferably until about 40%. This step is not indispensable for good functioning, but allows to economise in a marked way the chemical reagents, increases the yield of recirculation and reduces the cost of exploitation. To obtain a better yield, a flocculating agent such as Zetag 32 in 3 can be added. The separated water can be re-utilised 15 for the gas washing (scrubbing) produced by blast furnaces. The thickener 7 is constructed in such a way that a content of dry matter of about 30% to 50% can be obtained. To achieve thickening on the industrial scale, a centrifuge press such as Alfa Laval DSNX 4850, for example, can be used. Referring to the examples, one must take into account that the sludge deriving from lagooning does not undergo thickening like sludge entering because of the fact that the content in solid matter (about 15%) is sufficiently high. The sludge from blast furnaces A and B is made thick with the aid of a thickener. To this sludge there may be added a quantity of sludge which is easy to pump and which mixes well in the reactor. The thickened sludge, namely 2.5 tonnes/h of moist sludge or 1.5 tonne/h of dry sludge, is brought into the reactor 10 simultaneously with the leaching agent 2 (preferably HCI) and possibly with the "recirculate" 4 as well as an oxidising agent. This causes the passage of contaminating metals in the solution under the influence of the acid and the oxidiser. The ratio L/S is 10 and the pH in the reactor is lower than 2, optimally of the order of 1.5. At this pH, Zn and Pb pass into solution, while Fe is only partially dissolved. The redox potential is preferably higher than 450 mV. If the time spent in the reactor is of the order of about 2 hours, a quantitative dissolution of Zn and Pb occurs. Downstream from the reactor 10, the suspension is filtered by means of a filter in 6, possibly after addition 8 of a flocculating agent (for example Zetag 32). The filtration web can be washed by water 15; the residue is a filtration cake which contains essentially carbon and iron and can be recovered by bringing it to the sintering installation 16, the water being recirculated from 17 to 1 while undergoing neutralisation 18 by a lime sludge or NaOH, a part 19 being added to the filtrate 20. Thereafter, 3.1 tonnes of sludge treated with 30% of dry matter are sent to the sintering installation in operation conditions. The filtrate 20 is then separated on an ion exchanger 7 in order to eliminate the dissolved Zn and Pb. Only Zn and Pb undergo exchange on an anion exchanger, while other metals (Ca, Al, Mg, Mn and Fe) do not undergo any exchange and simply pass into the columns while remaining in solution. Downstream from the ion exchanger, the solution can be oxidised in the oxidisation reactor 9 (for example with the aid of NaOCI or Ca(OC) 2 fed in 21) and the "recirculate" 4 can be re-utilised in the leaching reactor 10. The "recirculate" 4 after oxidisation contains essentially a solution of FeCl 3 which causes in the reactor the precipitation of Fe.++ in the form of goethite and makes possible the re-use of hydrochloric acid released by this reaction as a reagent. After saturation of the ion exchanger 7, a regeneration with the help of water coming from a buffer reservoir 11 is necessary. Regeneration is possible with water or with the "regenerate", namely with Zn and Pb in solution. The Zn in the regenerate can undergo purification in the "cementation" installation 12. This occurs by adding a quantity of metallic Zn in powder form which will pass into solution, while the Pb present precipitates as metal. In this way, one obtains a Zn solution which is sufficiently pure and concentrated to make possible a good recovery of the metal. This recovery can be effected for example by electrolysis 13 (which produces the metal) or a precipitation reaction 14 (which produces zinc hydroxide or zinc carbonate). A leaching reactor 10 which could be appropriate is represented in figure 3. In the lower part 120 of the reactor, there is fed in the entering sludge and acid 121 (possibly "recirculate"). The suspension having reacted is emptied into 115 coming from the overflow canal 122. A barrier ring 123 can be provided to prevent the scum that has formed passes into the overflow. The suspension can be moved by gravity towards a buffer reservoir or directly towards a band filter 6 in figure 2. In the figure 4 which is mentioned in example 8, the sludge entering 115 is brought to a filtering web with curl strands 117 and the filtrate is taken off, for example by vacuum pumps 118 and 120 respectively towards 20 and 17, 19 (see figure 2), while the remaining solid matter 119, after having been washed in water, is finally separated by scratching the filtering web 117 at the end of the band. In the examples 9 to 13, there is used each time a reactor as in figure 3. It is possible to work with other types of reactor. Example 9: Leaching of sludge from a lagooning with the aid of HCl and FeC 3 without recirculation of the filtered "recirculate" This experiment comprises a succession of 8 tests. Example 10: Leaching of sludge from a lagooning with the aid of HCI and FeCI 3 , with recirculation of the filtered "recirculate" The recirculate is oxidised with NaOCI before being sent back to the reactor. This experiment comprises a succession of 9 tests. Example 11: Leaching of sludge from a laqooning with the aid of HCI and NaOCI, with recirculation of the filtered "recirculate" This experiment comprises a succession of 24 tests. Example 12: Leaching of sludge from a blast furnace A with the aid of HCI and NaOCI, with recirculation of the filtered "recirculate" This experiment comprises a succession of 11 tests. UALi< Example 13: Leaching of sludge from a blast furnace B with the aid of HCI and NaOCI, with recirculation of the filtered "recirculate" This experiment comprises a succession of 8 tests. The yields of leaching and the operation conditions are each time resumed as average values obtained for a type of sludge in conditions of working given as defined in the examples. Thus, in example 9, the value is the average value of 9 tests. The difference indicated is the standard deviation from the mean. The table 9 is a representation of the average yields of leaching for Zn, Pb and Fe in various tests, indicating also the average pH and the redox potential. Example 9 10 11 12 13 pH labo 1.35+0.26 1.33+0.23 1.18+0.36 0.59+0.27 1.11+0.52 E (mV) 496+4 510 3.5 694+79 706 141 704+57 Zn (%) 95.4+1.4 96.1+2.0 96.1 +1.3 95.9 1.5 98.1.2 +1.2 Pb (%) 96.9+1.2 97.6+1.3 93.8+4.7 92.5+5.6 96.5+2.9 Fe(%) 45.6+8.5 51+20 32+10 41+22 49+34 Table 9: representation of average yields (%) for Zn, Pb and Fe and for pH of the redox potential (mV) of the sample which has been filtered in the laboratory. NT 0T

Claims (13)

1. Process for the leaching of non-ferrous metals such as Zn and Pb to-separate them from sludges containing iron, comprising the following operation steps: - a non-selective step of leaching of sludge containing iron in an acidic oxidising medium at a pH lower than 2 in a reactor in order to obtain a leached suspension; - a first step of separation in which the leached suspension is separated in a solid fraction and a solution; - a oxidisation step in which the solution undergoes oxidisation by an oxidising agent, causing the conversion of Fe++ present in the solution into Fe+++; - re-use of the oxidised solution in the leaching step, causing precipitation of the Fe+++ in the reactor and its elimination from the solution in the course of the first step of separation, while the bound acid in the form of FeC is recovered.

2. Process according to claim 1, comprising: - a second step of separation which includes a separation from the solution resulting from the first step of separation on at least one ion exchanger and in which the concentration in ions Cl - in the solution is at least about 1 M.

3. Process according to claim 1 or 2, wherein the first step of separation is carried out by filtration.

4. Process according to any of the preceding claims, wherein the step of leaching comprises the addition of HCI and/or FeCl 3 .

5. Process according to any of the preceding claims, wherein the pH at the time of the step of leaching is lower than a value of the order of 1.5.

6. Process according to any of the preceding claims, wherein the redox potential in the leaching step is higher than about 450 mV.

7. Process according to any of the preceding claims, wherein the oxidising agent is C12.

8. Process according to any of the preceding claims, wherein an average treatment time for the step of leaching is of about 2 hours.

9. Process according to any of the preceding claims, wherein the concentration of Cl - in the solution is preferably comprised between 1.5 and 2.5 mol/l.

10. Process according to any of the preceding claims, wherein the ion exchanger is regenerated by water and/or an aqueous solution of metallic ions, the outflowing "regenerate" being collected.

11. Process according to claim 10, wherein the Pb of the outflowing "regenerate" is separated by means of cementation.

12. Process according to claim 11, wherein the Zn of the "regenerate" is recovered by electrolysis.

13. Process according to claim 11, wherein Zn(OH) 2 or ZnCO 3 is recovered from the "regenerate" by means of a precipitation reaction.