PROCESS FOR REDUCING THE AMOUNTS OF ZINC (ZN) AND LEAD (PB) IN MATERIALS CONTAINING IRON (FE)

Abstract

The present invention relates to a process for reducing the amounts of zinc and lead in starting materials comprising iron which comprises the steps of: selectively leaching Zn and Pb comprised in the starting materials by mixing the starting materials with hydrochloric acid and an oxidizing agent comprising at least 5 wt-% of manganese dioxide in one or several reactor(s) at a temperature superior or equal to 35 C. and at a p H comprised between 0.5 and 3.5, filtrating the mixture obtained in order to separate the solid and the filtrate, washing the solid with water, the resulting solid comprising mainly Fe, a reduced amount of Zn and Pb compared to the original starting materials, recovering the filtrate of step b) and the washing water of step c) which comprise chloride, solubilized Zn and Pb in one or several reactor(s), precipitating solubilized Zn, Pb in the recovered filtrate and the washing water by mixing with a neutralizing agent, filtrating and washing the solid residues obtained in step e) in order to remove the chloride from the solid residues which comprise at least Pb and Zn. The present invention also refers to the use of the materials obtained after treatment in a in a sinter plant and blast furnace or in all pyrometallurgical furnace which value iron such as electrical arc furnace (EAF), cupola furnace, oxycup furnace, submerged arc furnace (SAF), a plasma furnace, rotary hearth furnace.

Claims

1.-16. (canceled)

17. A process for reducing the amounts of zinc and lead in starting materials comprising iron, which comprises the steps of: a) selectively leaching Zn and Pb comprised in the starting materials by mixing the starting materials with hydrochloric acid and an oxidizing agent in one or several reactor(s) at a temperature superior or equal to 35 C. and at a pH comprised between 0.5 and 3.5, b) filtrating the mixture obtained in order to separate the solid and the filtrate, c) washing the solid with water, the resulting solid comprising mainly Fe and a reduced amount of Zn and Pb compared to the original starting materials, d) recovering the filtrate of step b) and the washing water of step c) which comprise chloride, solubilized Zn and Pb in one or several reactor(s), e) precipitating solubilized Zn, Pb in the recovered filtrate and the washing water by mixing with a neutralizing agent, f) filtrating and washing the solid residues obtained in step e) in order to remove the chloride in solution from the solid residues which comprise at least Pb and Zn, wherein the oxidizing agent of step a) comprises at least 5 wt-% of manganese dioxide.

18. The process according to claim 17, wherein manganese dioxide represents 10 to 100 wt-% of the total amount of oxidizing agent in step a).

19. The process according to claim 17, wherein manganese dioxide represents 20 to 80 wt-% of the total amount of oxidizing agent in step a).

20. The process according to claim 17, wherein manganese dioxide represents 25 to 50 wt-% of the total amount of oxidizing agent in step a).

21. The process according to claim 17, wherein the oxidizing agent of step a) comprises one or more further oxidizing agents being selected from the group consisting of: air, oxygen, ozone, O.sub.2 enriched air, chlorine, hypochlorite, chlorite, chlorine dioxide, chlorate, perchlorate, bromine, potassium permanganate, nitric acid, dichromic acid, hydrogen peroxide and any combination thereof.

22. The process according to claim 17, wherein the starting materials before the treatment comprise: from 1 to 55 wt-% of Fe, from 0 to 65 wt-% of C, from 0.01 to 30 wt-% of Zn, and from 0.01 to 5 wt-% of Pb, based on the total weight of the starting materials.

23. The process according to claim 17, wherein the treated materials comprise: from 0.9 to 60 wt-% of Fe, from 0 to 70 wt-% of C, less than 0.3 wt-% of Zn, less than 0.1 wt-% of Pb, and less than 1 wt-% of Cl, based on the total weight of the treated materials.

24. The process according to claim 17, wherein the starting materials further comprise carbon.

25. The process according to claim 17, wherein the leaching step a) is performed in a series of reactors.

26. The process according to claim 17, wherein the neutralizing agent of step e) is selected from the group consisting of: lime, hydrated lime, limestone, milk of lime, milk of limestone, caustic soda, potassium hydroxide, sodium carbonate, BOF slag, the starting materials, and any combination thereof.

27. The process according to claim 17, wherein it comprises an additional step a) between the steps a) and b) which consists in a selective precipitation of iron.

28. The process according to claim 27, wherein the selective precipitation step a) of iron comprises the addition of an oxidizing agent which is selected from the group consisting of: air, oxygen, ozone, O.sub.2 enriched air, chlorine, hypochlorite, chlorite, chlorine dioxide, chlorate, perchlorate bromine, manganese dioxide, potassium permanganate, nitric acid, dichromic acid, hydrogen peroxide and any combination thereof, and the addition of a neutralizing agent which is selected from the group consisting of: lime, hydrated lime, limestone, milk of lime, milk of limestone, caustic soda, potassium hydroxide, sodium carbonate, BOF slag, the starting materials and any combination thereof, in the mixture obtained after the leaching step a).

29. The process according to claim 26, wherein O.sub.2, O.sub.2 enriched air and/or air are injected to the recovered filtrate and washing water during the precipitation step e).

30. The process according to claim 17, wherein Fe and manganese are solubilized at step a) and precipitated at step e).

31. The process according to claim 17, wherein the pH at step e) is between 7 and 12.

32. The process according to any of claims 17, wherein the starting materials to be treated are blast furnace sludge, blast furnace dust, BOF sludge or BOF dust.

33. The process according to claim 17, further comprising using the obtained treated materials obtained in a sinter plant, a blast furnace or in a pyrometallurgical furnace which values iron.

34. The process according to claim 17, wherein the pyrometallurgical furnace is selected from electrical arc furnace (EAF), cupola furnace, oxycup furnace, submerged arc furnace (SAF), plasma furnace, and rotary hearth furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0118] Preferred embodiment of the invention will now be described, by way of an example, with reference to the attached drawings namely:

[0119] FIG. 1 is a scheme of one embodiment of the process.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0120] FIG. 1 describes an embodiment of the process according to the present invention. The starting material to be treated is a blast furnace sludge 1 which corresponds in fact to the sludge rejected at the outlet of the gas cleaning plant.

[0121] The process can be divided in two major steps: step n1 corresponding to the treatment of the blast furnace sludge and step n2 corresponding to the recovering of the solubilized metals.

[0122] The treatment of the blast furnace sludge 1 consists in selectively removing the undesirable contaminants from the ferrous matrix namely: Zn and Pb. This is achieved by submitting the blast furnace sludge 1 to a leaching step 2 taking place preferably in a series of agitated reactors working in cascade in order to better control the pH and the addition of the oxidizing agent. The reaction takes place at a temperature of about55 C. This leaching step 2 is carried out with the addition of an oxidizing agent which is a mixture of MnO.sub.2 and O.sub.2 3 and a solution of HCl 4. Preferably, the molar ratio between oxidizing agent (Mn)/Zn is from 0.1 to 5 and the concentration of HCl is adjusted in order to decrease the pH between 1 and 2. The mixture goes by overflowing from reactor to reactor. The leaching step 2 is conducted for 30 to 60 minutes. These conditions allow to promoting the leaching of Zn and Pb compared to the other present metals.

[0123] At the end of the leaching step 2, due to the effect of the addition of the oxidizing agent which is a mixture of MnO.sub.2 and O.sub.2 3 and HCl 4, Zn and Pb are solubilized within the liquid phase under chloride form. Furthermore, Mn coming from MnO.sub.2, is also in solubilized form Mn.sup.2+ in liquid phase. A limited quantity of Fe may also be solubilized during this leaching step 2.

[0124] Regarding Fe, it is preferred to precipitate it under Fe III form. Indeed, the precipitation of Fe II is done at a pH value which induces the co-precipitation of Zn and Pb. That is why, in order to keep Zn and Pb in solution, Fe has to be precipitated under Fe III, which have a precipitation pH lower than the precipitation of Zn and Pb in such chloride media. As a result, an optional step corresponding to the selective precipitation of Fe 5 may be implemented. This selective precipitation of Fe 5 consists in the addition of a neutralizing agent which is CaO and the injection of air and/or O.sub.26 in the mixture. The addition of air and/or O.sub.2 and CaO 6 leads to the oxidation and the precipitation of Fe under a mixture of goethite form FeOOH and hydroxide form Fe(OH).sub.3.

[0125] Then, a filtration step7 is implemented in order to separate the liquid phase from the solid. The liquid phase corresponding to the filtrate comprises solubilized salts of Zn, Pb, Fe, Mn and chloride. The solid comprises Fe, C, and only few traces of Zn, Pb and Cl. The filtration step 7 is implemented by using a filter press or vacuum belt filter where liquids are removed from the solid. In case of using filter press, a storage pulp tank and a filtrate storage reactor may be preferably foreseen before and after the filter press in order to assure a continuous process because of the batch mode of the filter press. The filtered solid, which forms a cake, is then submitted to a washing step 8 with water on the filter in order to remove the remaining impregnated salts, namely the salts which were comprised in the filtrate. Preferably the washing step 8 is carried out with hot water having a temperature superior or equal to 35 C. and more preferably superior or equal to 50 C. This allows maintaining the lead solubilized in solution and preventing the crystallization of PbCl.sub.2 in the treated blast furnace sludge 9 (corresponding to Fe by-product).

[0126] At the end of step n1, the treated blast furnace sludge 9 comprising about less than 0.3 wt-% of Zn less than 0.1 wt-% of Pb and less than 1 wt-% of CI is obtained. This treated blast furnace sludge 9 can be directly recycled for use, for example, in sinter plant.

[0127] Step n2 corresponding to the recovering of the solubilized metals is preferably implemented in another series of agitated reactor in order to better control the pH. The filtrate and the washing water are recovered at the end of step n1 and put into a reactor. This mixture goes by overflowing from reactor to reactor. In order to achieve the precipitation of the solubilized metals (Zn, Pb, Fe and Mn), the pH of the liquid mixture is increased preferably until 8. This is achieved by the precipitation step 10 which consists in the addition of a neutralizing agent such as milk of lime (Ca(OH).sub.2) 11 in order to reach a pH preferably between 8 and 9. During the precipitation step 10, it should be preferably ensured that dissolved iron is under Fe III form and not Fe II form in order to prevent the formation of a gel when Fe precipitates under Fe(OH).sub.2 form. Thus, an oxidizing agent and preferably a mixture of air and/or O.sub.2 12 is injected within the reactor during the neutralization step.

[0128] At the end of the precipitation step 10, the metals Zn, Pb, Fe precipitate under hydroxide form whereas solubilized manganese is oxidized from Mn.sup.2+ to Mn.sup.4+ due to the effect of the injection of air and/or O.sub.2 12 and thus precipitates under MnO.sub.2 form.

[0129] The precipitate is then separated from the liquid phase through a filtration step 13. This filtration step 13 is preferably performed with a filter press working in batch mode. In order to assure the continuous flow of the process, storage reactor before and after the filter may be foreseen. The filtered precipitate is thereafter submitted to a washing on the filter step 14 with water in order to remove the impregnated salts mainly composed by chloride. At the end of the filtration step 13 and washing step 14, a salty liquid effluent 15 with a concentration from about 30 to 40 g/l of Cl is produced.

[0130] The filtered precipitate corresponds to solid residues 16 (namely Zn by-product) of Fe, Zn, Pb and Mn.

[0131] These solid residues can be recycled e.g. in non-ferrous industry.

[0132] Example: For treating 1 ton of blast furnace sludge with a composition of Fe 25%, C 45%, Zn 3.5% and Pb 0.5%, 0.50 ton of HCl 32% may be added in order to reach a pH about 1.5 and a quantity of manganese (IV) with a molar ratio between Mn/Zn of about 0.6. At the end of step n1, about 0.9 ton of treated blast furnace sludge (Fe by-product), which forms a cake, comprising about 26 wt-% of Fe, 50 wt-% of C, less than 0.3 wt-% of Zn, less than 0.1 wt-% of Pb and less than 0.5 wt-% of Cl is obtained. This content of CI is obtained with a washing on filter press with a quantity of water of 1-2 l/kg of Fe/C. The quantity of solution recovered is 4 m.sup.3 (filtrate+washing water).

[0133] In order to recover the solubilized metals, the solution of filtrate and washing water is then submitted to a precipitation step. The quantity of milk lime 25% used to reach pH 9 and to precipitate all the metals solubilized in the solution (filtrate and washing water) is preferably around 720 kg. The resulting mixture is then filtrated and washed with a quantity of water of 1-2 l/kg of Zn/Pb sludge. The filtered precipitate corresponds to solid residues (Zn by-product) of Fe, Zn, Pb and Mn. In particular, about 0.15 ton which comprises about 9 wt-% Fe, 20 wt-% Zn and 3 wt-% Pb is recovered. At the end of the filtration and washing step, about 5 m.sup.3 of salty liquid effluent with a concentration of about 30 g/l of Cl is produced.