PROCESS FOR HEAVY METAL REMOVAL FROM IRON- AND STEELMAKING FLUE DUST

Abstract

A process, for the selective heavy metal removal from iron- and/or steelmaking flue dust, including steps of: preparing a feedstock (FS) by blending or mixing a chloride precursor material (CPM) and ironmaking and/or steelmaking flue dust including heavy metals (ISFD), the heavy metals including Pb and Zn and optionally Cd; in a first reaction step in a first reactor reacting the CPM with the ISFD by thermal treatment of the FS at a temperature in a range of 700 C. to 950 C. removing at least 70 wt. % of Pb from the ISFD; in a subsequent second reaction step in a second reactor further reacting the CPM with the ISFD by thermal treatment of the feedstock FS at a temperature in a range of 850 C. to 1200 C.; and obtaining a solid material after the second reaction step. The invention also relates to a plant implementing the process.

Claims

1. A process for the selective heavy metal removal from iron- and/or steelmaking flue dust (ISFD), the process comprising the steps of: preparing a feedstock (FS) by blending or mixing a chloride precursor material (CPM) and ironmaking and/or steelmaking flue dust comprising heavy metals (ISFD), the heavy metals being at least Pb and Zn and optionally also Cd; in a first reaction step in a first reactor reacting the CPM with the ISFD by thermal treatment of the FS at a temperature in a range of 700 C. to 950 C. removing at least 70 wt. % of Pb from the ISFD by chlorination and evaporation of the Pb and removing it from the first reactor via the off-gas formed; in a subsequent second reaction step in a second reactor further reacting the CPM with the ISFD by thermal treatment of the feedstock FS at a temperature in a range of 850 C. to 1200 C. removing the Zn from the ISFD by chlorination and evaporation of the Zn and removing it from the second reactor via the off-gas formed; and obtaining and removing a secondary solid material after the second reaction step from the second reactor.

2. The process according to claim 1, wherein during the first reaction step at least 70% of the Pb is removed from the ISFD by evaporating PbCl.sub.2 at temperatures from 700 C. to 950 C.

3. The process according to claim 1, wherein during the first reaction step at least 80 wt. % of the Pb is removed from the ISFD.

4. The process according to claim 1, wherein during the second reaction step the zinc is removed from the ISFD by evaporating ZnCl.sub.2 at temperatures from 850 C. to 1200 C., under a non-oxidizing atmosphere.

5. The process according to claim 1, wherein off-gasses generated in the first reaction step in the first reactor are treated in an off-gas treatment system to obtain lead-enriched flue dust.

6. The process according to claim 1, wherein off-gasses generated in the second reaction step in the second reactor are treated in an off-gas treatment system to obtain zinc-enriched flue dust.

7. The process according to claim 1, wherein the ratio of ironmaking and/or steelmaking flue dust comprising ISFD and CPM being chosen so that the chloride content of the FS is between 100% and 150%, preferably between 100% and 130%, of the amount necessary for a stoichiometric conversion of the heavy metals in the ISFD into chlorides.

8. The process according to claim 1, wherein the chloride precursor material (CPM) is selected from the group comprising consisting of polyvinyl chloride (PVC), waste comprising PVC, chlorinated rubber or other chlorinated polymers, FeCl.sub.2, FeCl.sub.3, and CaCl.sub.2).

9. The process according to claim 1, wherein the chloride precursor material (CPM) is FeCl.sub.2.

10. The process according to claim 1, wherein each of the first reaction step and the second reaction step are performed in a rotary kiln.

11. A method of use of the obtained secondary solid material after the second reaction step according to claim 1, comprising feeding the obtained secondary solid material to an ironmaking operation as a ferrous raw materials and/or carbon resource.

12. A method of use of the zinc-enriched flue dust from the second reaction in the second reactor according to claim 6, comprising feeding the zinc-enriched flue dust to a hydrometallurgical process or electrolytic process to recover the zinc metal or zinc compounds.

13. A plant for implementing the process for the selective heavy metal removal from iron- and/or steelmaking flue dust according to claim 1, the plant comprising an apparatus for preparing a feedstock FS by blending or mixing the CPM and ISFD; a first reactor configured for the first reaction step and the first reactor being equipped with an off-gas treatment system; and a second reactor configured for the second reaction step and the second reactor being equipped with an off-gas treatment system.

14. The plant according to claim 13, wherein the first reactor and the second reactor are positioned in-line.

15. The plant according to claim 13, comprising at least one operation selected from the group comprising a blast furnace operation, a direct reduced iron making operation, a reducing electrical furnace operation, an electric arc furnace operation, a HIsarna-type ironmaking process, and a basic oxygen steelmaking operation; the operation(s) including off-gas generation and being equipped with an off-gas treatment system to capture the flue dust comprising heavy metals, the heavy metals being at least lead, zinc and optionally also cadmium (ISFD); the apparatus, the first reactor and the second reactor; the ISFD being blended or mixed in apparatus with a chloride precursor material (CPM) to prepare a feedstock (FD); subjecting said feedstock (FD) to a thermal treatment in the first reaction step in the first reactor, and the subsequent second reaction step in the second reactor; and the obtained secondary solid material after the second reaction step and removed from the second reactor is re-used in an ironmaking operation as ferrous containing raw material via direct injection or after agglomeration.

16. The process according to claim 1, wherein during the first reaction step at least 70% of the Pb is removed from the ISFD by evaporating PbCl.sub.2 at temperatures from 800 C. to 900 C., under a non-oxidizing atmosphere.

17. The process according to claim 1, wherein during the first reaction step at least 90 wt. % of the Pb is removed from the ISFD.

18. The process according to claim 1, wherein during the second reaction step the zinc is removed from the ISFD by evaporating ZnCl.sub.2 at temperatures from 900 C. to 1100 C., under a non-oxidizing atmosphere.

19. The plant according to claim 13, wherein the first reactor and the second reactor are positioned in-line, and each of the first reactor and the second reactor are a rotary kiln.

Description

DETAILED DESCRIPTION OF THE FIGURE

[0060] The invention will now be explained by means of the following, non-limiting FIGURE.

[0061] FIG. 1 shows schematically the process flow of the process according to the invention. The ironmaking and/or steelmaking flue dust 15 comprising heavy metals (ISFD), notably at least zinc, lead and optionally also cadmium, and originating from one or more operations including a blast furnace (BF) operation 13, a direct reduced iron (DRI) making process 12, a reducing electrical furnace (REF) operation 10, an electric arc furnace (EAF) operation 10, a HIsarna-type ironmaking process 14, and a basic oxygen steelmaking operation (BOF) 11, is mixed or blended in an apparatus 3 with a chloride precursor material (CPM) 16, for example FeCl.sub.2, to prepare a feedstock (FD), for example in the form of pellets.

[0062] The feedstock is feed into a first reactor 1 (in this embodiment a rotary kiln) to react in a first reaction step the CPM 16 with the ISFD 15 by thermal treatment of said feedstock at a temperature in a range of 700 C. to 950 C., and with preferred narrower temperature ranges, removing at least 70 wt. % of lead present from the ISFD by chlorination and evaporation of PbCl.sub.2 via the off-gas formed. The formed off-gas comprising the PbCl.sub.2 during the first reaction step is cleaned in a cleaning step using an off-gas treatment system 4 such that a solid lead-enriched residue or flue dust 5 is obtained having a very high fraction of lead and other heavy metals like cadmium and mercury together with some zinc. This highly heavy-metal enriched solid residue or flue dust 5 has no immediate commercial value and is carefully stored in a landfill against high costs. However, the total amount of heavy-metal enriched solid residue or flue dust 5 is still significantly smaller compared to the outputted solid residue originating from for example the process disclosed in WO2019/043261-A1 where the ISFD is being treated in a single thermal treatment such that following off-gas treatment the resultant flue dust or solid residue is enriched with the evaporated zinc but also with substantial higher amounts of the lead, cadmium and other toxic non-ferrous heavy metals originally present in the ISFD. The high contamination levels of lead and other toxic non-ferrous heavy metals renders such solid residue unsuitable for re-use in the zinc recovery industry and it has to be discarded via landfilling or otherwise.

[0063] In accordance with the invention, next the feedstock outputted from the first reaction step is subjected to a subsequent second reaction step in a second reactor 2 (in this embodiment a rotary kiln) positioned in-line to the first reactor to further react the CPM 16 with the ISFD 15 by thermal treatment of the feedstock at a temperature in a range of 850 C. to 1200 C., and with preferred narrower temperature ranges, removing substantially most of the zinc present in the ISFD by chlorination and evaporation of ZnCl.sub.2.

[0064] The off-gas of the second reaction is cleaned in a cleaning step using another off-gas treatment system 6 such that a zinc-enriched solid residue or flue dust 7 is obtained having a very high weight fraction of zinc compounds. The zinc-enriched solid residue or flue dust 7 is of high purity as the presence of other heavy non-ferrous metals in particular lead, cadmium and mercury is very low as most of these have already concentrated in the lead-enriched solid residue or flue dust 5 originating from the first reaction step. The zinc-enriched solid residue or flue dust 7 has a high added value and can be readily re-used in a hydrometallurgical process (not shown) or electrolytic process (not shown) or another zinc recovery process as known in the art of zinc recovery to recover the zinc metal or zinc compounds. The output of the second reaction is so-called a secondary solid material 8 having very low levels of non-ferrous heavy metals and having high fractions of metallic iron, some remaining carbon, non-reduced FeOx and inert gangue minerals, and can be readily re-used in an ironmaking operation 10, 12, 13, or 14 as a ferrous raw materials and/or carbon resource, either via direct injection or after agglomeration, for example as pellets or briquettes, and thereby avoiding the disadvantageous accumulation of lead and other toxic heavy metals in an ironmaking operation.

[0065] The invention will now be illustrated with reference to non-limiting embodiments according to the invention.

Example

[0066] Blast furnace flue dust origination from an industrial ironmaking operation and for which the composition (dry mass) is listed in Table 1 has been pelletized into a feedstock of micro-pellets having a diameter in a range of 3 to 5 mm using about 0.5 wt. % bentonite as a binder material and about 4.6 wt. % FeCl.sub.2 as chloride precursor material originating as by-product from steel pickling. The composition of the micro-pellets is listed also in Table 1. All compositions listed in Table 1 have been determined using thermogravimetric analysis (TGA) and inductively coupled plasma (ICP) techniques well known to the person skilled in the art.

[0067] A small batch of 15 gram of these pellets has been heat-treated on a laboratory scale in a quartz glass tube under a non-oxidizing flowing inert nitrogen atmosphere in a first reaction step for 1 hour at 800 C. for chlorination and evaporation of at least the lead and cadmium and some of the zinc. The flue dust from this first reaction step is Pb-rich with about 4.54 wt. % of lead and 15.43 wt. % of CI as listed in Table 1. The flue dust is also enriched with Cd. Given the presence of high levels of toxic heavy metals, this flue dust as by-product of the first reaction step has no commercial value in an iron- and/or steelmaking operation. The micro-pellets or solid residue (or feedstock) after this first reaction step have a very low lead content of about 0.01 wt. % and still about 2.14 wt. % zinc remaining compared to the 2.73 wt. % zinc in the original BF flue dust. The output of the first reaction step is used as input for a subsequent second reaction step under a non-oxidizing flowing inert nitrogen atmosphere by holding said solid material for 1 hour at 1000 C. for chlorination and evaporation of the zinc and remaining other heavy metals. The flue-dust from this second reaction step is Zn-rich with about 17.26 wt. % of zinc and has a very low lead content of about 0.02 wt. % as listed in Table 1, and may form a valuable by-product for use in the zinc recovery industry. The secondary solid residue 8 obtained after the second reaction step is both very low in zinc and lead content of respectively 0.04 wt. % and 0.01 wt. %, and is also free from Cd. Whereas the original BF flue dust has about 2.73 wt. % zinc and about 0.54 wt. % lead. Due to its high iron content, high carbon content and very low zinc and lead contents the solid residue obtained after the second reaction step can be re-used in an ironmaking process.

[0068] The calculated mass balance of the experiments in accordance with the invention is: [0069] Residual solid mass after 800 C.: 86%, [0070] Gas produced at 800 C.: 13%, [0071] Residual solid mass after 1000 C.: 72%, [0072] Gas produced at 1000 C.: 25%.

[0073] Where previously the BF flue dust due to its composition had to be discarded, using the process according to the invention at least two useable by-products are extracted from this BF flue dust, namely the flue dust 7 from the second reaction step and the secondary solid residue 8 obtained after the second reaction step, and thereby very significantly reducing the total amounts of ironmaking by-products that have to be discarded.

TABLE-US-00001 TABLE 1 Compositions of input and output materials at various stages of the chlorination process in accordance with the invention. Compositions are in wt. %. Element C Ca Cd Fe Mg Mn Pb Flue dust 40.95 1.65 0.01 24.70 0.59 0.10 0.54 Pellet 38.95 1.59 0.01 24.75 0.57 0.10 0.51 Pb-bearing dust 13.66 0.17 0.05 6.37 0.24 0.01 4.54 Solid residue 43.75 1.83 0.00 28.10 0.64 0.11 0.01 after step 1 Zn-bearing dust 34.32 1.86 0.01 4.54 0.22 0.07 0.02 Solid residue 47.25 1.92 0.00 32.90 0.73 0.12 0.01 after step 2 Element S Zn Si Al Cl P Flue dust 1.11 2.73 2.50 1.48 0.54 0.17 Pellet 1.06 2.60 2.50 1.45 2.08 0.16 Pb-bearing dust 0.53 6.87 2.52 1.03 15.53 0.92 Solid residue after step 1 1.17 2.14 2.60 1.56 0.43 0.07 Zn-bearing dust 1.01 17.26 3.46 1.03 1.74 0.05 Solid residue after step 2 1.25 0.04 2.60 1.71 0.26 0.07

[0074] The above-discussion is intended to be merely illustrative of the present process and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Accordingly, the specification and drawing are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawing, the disclosure, and the appended claims. The mere fact that certain measures are recited in different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the appended claims.

LIST OF REFERENCE NUMBERS

[0075] 1. first reactor; [0076] 2. second reactor; [0077] 3. blender/mixer; [0078] 4. off gas treatment system connected to the first reactor; [0079] 5. lead-enriched flue dust; [0080] 6. off gas treatment system connected to the second reactor; [0081] 7. zinc-enriched flue dust; [0082] 8. secondary solid material; [0083] 10. reducing electrical furnace operation; [0084] 11. basic oxygen steelmaking operation; [0085] 12. direct reduced iron making process; [0086] 13. blast furnace operation; [0087] 14. HIsarna-type ironmaking process; [0088] 15. ironmaking and/or steelmaking flue dust; [0089] 16. chloride precursor material;